UNIVERSITY OF WISCONSIN-LA CROSSE

Graduate Studies

AN ANALYSIS OF FOUR DIMENSIONAL STEREOTACTIC BODY RADIATION

THERAPY FOR LUNG CANCER: ABDOMINAL COMPRESSION VERSUS FREE

BREATHING

A Research Project Report Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Medical Dosimetry

Robert Anthony Rostock Jr.

College of Science & Health

Medical Dosimetry Program

May 2013

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AN ANALYSIS OF FOUR DIMENSIONAL STEREOTACTIC BODY RADIATION

THERAPY FOR LUNG CANCER: ABDOMINAL COMPRESSION VERSUS FREE

BREATHING

 

 

By  Robert Anthony Rostock Jr.  

 

 

 

We recommend acceptance of this project report in partial fulfillment of the candidate's requirements for the degree of Master of Science in Medical Dosimetry

The candidate has met all of the project completion requirements.

     

 

 

         

Nishele Lenards, M.S. Date

Graduate Program Director    

April 30, 2013

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The Graduate School

University of Wisconsin-La Crosse La Crosse, WI

Author: Rostock, Robert A.

Title: An Analysis of Four Dimensional Stereotactic Body Radiation Therapy for Lung Cancer: Abdominal Compression versus Free Breathing

Graduate Degree/ Major: MS Medical Dosimetry

Research Advisor: Nishele Lenards, M.S.

Month/Year: May 2013

Number of Pages: 54

Style Manual Used: AMA, 10th edition

Abstract

Lung cancer can be particularly difficult to treat with radiation therapy due to the

motion of the tumor inside the lungs during the respiratory cycle. Advanced treatment

modalities such as stereotactic body radiation therapy (SBRT) allow a moving lung tumor

to be treated accurately and precisely. Stereotactic body radiation therapy can be

performed in a number of ways; with a linear accelerator, CyberKnife system, or helical

TomoTherapy. Linear accelerator based SBRT requires the use of complex

immobilization devices and advanced integrated imaging devices. Stereotactic body

radiation therapy can be performed while the abdomen is compressed, while the patient

breathes freely, or with respiratory gating. The aim of this study was to compare two

linear accelerator based SBRT methods: SBRT with abdominal compression and SBRT

with free breathing. The results of the study were inconclusive; one method did not

outperform the other outright with regard to metrics provided in the Radiation Therapy

Oncology Group (RTOG)-0236 protocol.1

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The Graduate School University of Wisconsin - La Crosse

La Crosse, WI

Acknowledgments

I would like to thank the staff where I completed clinical internship affiliated with

the University of Wisconsin - La Crosse Medical Dosimetry program. This research

would not have been possible without their knowledge and support.

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Table of Contents

.................................................................................................................................................... Page

Abstract ............................................................................................................................................3

List of Tables ...................................................................................................................................6

List of Figures ..................................................................................................................................7

Chapter I: Introduction ....................................................................................................................8

Statement of the Problem ...................................................................................................13

Purpose of the Study ..........................................................................................................13

Assumptions of the Study ..................................................................................................14

Definition of Terms ............................................................................................................14

  Limitations of the Study………………………………………………………………….18

Methodology ......................................................................................................................18

Chapter II: Literature Review ........................................................................................................20

Chapter III: Methodology ..............................................................................................................31

Sample Selection and Description .....................................................................................31

Instrumentation ..................................................................................................................31

Data Collection Procedures ................................................................................................32

Data Analysis .....................................................................................................................32

Limitations .........................................................................................................................32

Summary ............................................................................................................................33

Chapter IV: Results ........................................................................................................................34

Item Analysis .....................................................................................................................34

Chapter V: Discussion ...................................................................................................................38

Limitations .........................................................................................................................39

Conclusions ........................................................................................................................39

Recommendations ..............................................................................................................40

References ......................................................................................................................................51

 

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List of Tables

…………………………………………………………………………………………………Page

Table 1: Percent volume of Total Lung without PTV receiving 20 Gy .........................................41

Table 2: Total Lung without PTV Mean Dose ..............................................................................41

Table 3: Patient A treatment plan results .......................................................................................41

Table 4: Patient B treatment plan results .......................................................................................42

Table 5: Patient C treatment plan results .......................................................................................42

Table 6: Patient D treatment plan results .......................................................................................42

Table 7: Patient E treatment plan results .......................................................................................42

Table 8: Patient F treatment plan results ........................................................................................43

Table 9: Patient G treatment plan results .......................................................................................43

Table 10: Patient H treatment plan results .....................................................................................43

Table 11: Patient I treatment plan results ......................................................................................43

Table 12: Patient J treatment plan results ......................................................................................44

Table 13: Max dose 2 centimeters away from PTV (D2cm) .........................................................44

Table 14: Ratio of prescription isodose volume to the PTV ..........................................................44

Table 15: Low Dose Spillage .........................................................................................................45

Table 16: High Dose Spillage ........................................................................................................45

Table 17: Prescription isodose surface coverage ...........................................................................46

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List of Figures

Figure 1: Ratio of prescription isodose volume to the PTV ..........................................................47

Figure 2: High Dose Spillage .........................................................................................................49

Figure 3: Low Dose Spillage .........................................................................................................49

Figure 4: Prescription isodose surface coverage ............................................................................50

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Chapter I: Introduction

For both men and women, lung cancer is the leading cause of mortality among all

cancers.2 It is estimated that in 2013, more than 225,000 new cases of lung cancer and 155,000

deaths due to lung cancer are expected in the United States.3 Roughly 10% of all patients with

lung cancer are expected to live for at least 5 years after their diagnosis.4 It is assumed that the

primary cause of lung cancer is cigarette smoking. Ninety percent of all patients diagnosed with

lung cancer have a history of smoking.4 There is evidence to support that the number of

cigarettes smoked per day, the degree of inhalation, and the age of initiation of smoking are all

directly related to an increased risk of lung cancer.2

Roughly 10% of lung cancers are thought to be caused by carcinogens other than

cigarette smoking.2 Exposure to these carcinogens usually arises from occupational and

residential hazards. Occupations that involve exposure to agents such as arsenic, asbestos,

beryllium, chloromethylethers, chromium, hydrocarbons, mustard gas, nickel, and radiation

(including radon) significantly increase the risk of developing lung cancer.4 Although the

relationship between residential radon exposure and lung cancer is uncertain, it is still a concern

in the public eye. Increased risk of lung cancer has also been associated with diet, nutrition and

genetic factors.5 Adults with low fruit and vegetable intake are at a slightly higher risk for lung

cancer than those who have a normal intake.5 There are also genetic factors that help determine

how an individual will respond to lung carcinogens. The most common genetic factor associated

with an increased risk of lung cancer is mutation of the epidermal growth factor receptor

(EGFR).5 Other important genetic mutations associated with lung cancer include abnormalities in

genes encoding the Ras family of proteins, as well as retinoblastoma protein (Rb), protein 53

(p53), protein kinase B (Akt), liver kinase B1 (LKB1), and proto oncogene B-Raf (BRAF).5

There are two types of lung cancer: small cell lung cancer (SCLC) and non-small cell

lung cancer (NSCLC). Historically, treatments for lung cancer have varied based on the type of

cancer, stage of the tumor, and the patient’s current medical condition. Treatment options include

surgery, radiation therapy, chemotherapy, or a combined modality therapy. Small cell lung

cancer is typically treated with chemotherapy and radiation therapy due to the responsiveness

from both modalities. While NSCLC is curable, most patients die from disease progression due

to rapid development of drug resistance. The development of chemotherapy resistance often

causes NSCLC to metastasize to the central ipsilateral supraclavicular area.3 Despite the

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advancements in chemotherapy and radiation therapy, the survival rate for patients with NSCLC

has not changed in the past two decades.3

Radiation therapy is known to have great effect on SCLC, and is recommended to be

used concomitantly early in the course of chemotherapy. The objective with radiation therapy is

to treat all gross disease and possibly the ipsilateral hilum, bilateral mediastinal nodes, and the

ipsilateral supraclavicular area.4 Small cell lung cancer also has a tendency to metastasize to the

central nervous system (CNS). Because of this, prophylactic cranial irradiation is recommended

for patients with CNS metastases who have had a complete response to chemotherapy.4

For patients with NSCLC, the preferred treatment of choice is surgery. If the patient is

deemed inoperable or refuses surgery, then chemo-radiation is a potential curative alternative.

Assuming the patient is assessed to have operable NSCLC, there are a number of preoperative

and post-operative techniques involving radiation therapy and chemotherapy. Theoretically,

preoperative radiation therapy may improve respectability of larger tumors, sterilize cells beyond

the margins of resection, prevent dissemination by surgical manipulation, and allow the use of

lower doses of radiation postoperatively if needed.4 However, preoperative radiation alone is no

longer studied consistently due to more effective chemotherapy agents.4 The role of

postoperative radiation therapy is controversial; it has been shown to improve local control in

patients with residual disease, but has no proven benefit of increased survival rate for advanced

stage NSCLC.4

For inoperable locally advanced NSCLC, radical radiation therapy techniques with or

without chemotherapy are the primary alternative. This includes radiation therapy alone,

sequential chemotherapy and radiation, and concurrent chemoradiotherapy. Conventional

radiation therapy produced limited results when compared with surgery. This is due to inaccurate

tumor targeting and a lack of conformality of the dose distribution to the target volume.4 With

the advent of three dimensional conformal radiation therapy (3D-CRT), higher doses were

achievable, but at the cost of inducing late toxicities.4 For inoperable early-stage NSCLC, radical

radiation therapy incorporating advanced modalities is the preferred method. Stereotactic body

radiation therapy (SBRT) is a relatively new, radical radiation therapy technique that has proven

effective in the treatment of early stage NSCLC.

Stereotactic radiosurgery (SRS) is a radiation technique that delivers a single high dose of

radiation with high accuracy and precision to a target.6 The concept of SRS was developed in

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1951 by Lars Leksell, and the first prototype of Gamma Knife was installed in Sophiahemet in

1968.7 Stereotactic radiosurgery was first developed for intracranial lesions, and requires

complex immobilization devices. Stereotactic body radiation therapy evolved from a similar

concept as SRS, but was applied to lesions located outside of the brain. It was in the 1990s that

groups around the world attempted to mimic SRS to extracranial sites.7 From that point on,

centers around the world have conducted research on SBRT. Stereotactic body radiation therapy

has been found to largely exclude normal tissue in the high-dose region and optimize the

therapeutic ratio by increasing the effective dose to the tumor while excluding the normal tissue.6

Because of this, the number of fractions treated with SBRT can be limited to 5 or less. With the

availability of high speed computing and advanced imaging techniques that allow for improved

methods of dealing with respiratory motion, SBRT is becoming a growingly popular treatment

for spinal tumors, lung cancer, pancreatic cancer, and liver malignancies.

As mentioned previously, SBRT has become a promising treatment for early stage

NSCLC for patients who are ineligible for surgery or cannot tolerate surgery. It is also popular

for the avoidance of anesthesia and invasive procedures. Since NSCLC is predominantly a

disease in elderly patients with a history of smoking, which generally makes these patients

ineligible surgical candidates, SBRT can be particularly useful.8 However, SBRT comes with

considerable risk. The high dose of radiation given by SBRT can lead to severe toxicities. It is

crucial to achieve high quality treatment through meticulous planning to attain adequate tumor

coverage while avoiding organs-at-risk (OR) and maintaining reliable immobilization and

accurate tumor targeting and verification of dose delivery.8 And much like primary lung tumors,

SBRT has also become a popular treatment for lung metastases. Similar to primary lung tumors,

SBRT is very attractive as it avoids the risk of invasive procedures.9

As mentioned previously, SBRT requires strict patient immobilization and respiratory

motion control. It is imperative to minimize damage to OR by limiting the amount of tissue

included the high dose region that SBRT can create. There are a number of commercially

available immobilization devices, which include stereotactic body frames (with an additional

abdominal compression device), body cradles and vacuum bags. For cases involving targets that

are subject to respiratory motion, there are 3 methods of inducing motion control: motion

dampening, motion gating, and motion tracking.7

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The use of an abdominal compression device and active breathing control (ABC) are

examples of motion dampening. Abdominal compression is a method of inducing forced shallow

breathing (FSB).10 It is popular for SBRT of the lung because it reduces excursion of the

diaphragm and movement of a tumor while breathing. If a lung tumor is located in the lower

lobe, abdominal compression devices can reduce the excursion of the diaphragm by as much as 2

centimeters.6 This method significantly reduces tumor motion, but at the cost of less lung sparing

for tumors in the upper and middle lobe of the lung.11 Abdominal compression devices can be

uncomfortable for patients who already have a difficulties breathing due to lung cancer. In

addition to abdominal compression, ABC is a method to create a reproducible breath hold. An

ABC device is meant to suspend breathing at a predetermined position, but is normally used

during inhalation.10 An ABC system works by the patient breathing through an ABC device and

the operator specifying the appropriate lung volume and breathing cycle stage at which the

device will limit the patient’s breathing. Once the patient reaches the specified stage, the ABC

device activates and the patient can no longer inhale.

In motion gating, also known as respiratory gating, a fixed radiation beam is activated

only at a specific phase of the respiratory cycle.7 Motion due to the respiratory cycle is normally

tracked by a device placed on a patient’s body, close to the diaphragm. Motion tracking is an

invasive procedure where a fiducial marker is implanted within the target. A radiation beam then

tracks the fiducial marker, which moves in conjunction with the respiratory cycle. Four-

dimensional computed tomography (4DCT) can also be used with or without these methods of

motion control. Four dimensional computed tomography is useful in evaluating the mobility of a

tumor and can be used to generate an internal target volume (ITV) for treatment planning.7 In the

case that 4DCT is unavailable, CTs obtained at free breathing, deep inspiration, and deep

expiration can be effective in generating an ITV.

Along with the various immobilization devices and methods of motion control, there are

a number of ways that SBRT can be delivered. The current commercially available treatment

units that are able to deliver SBRT are all capable of image-guided radiation therapy (IGRT),

which makes target localization possible prior to treatment delivery.7 The most common form of

SBRT is linear accelerator based. Stereotactic body radiation therapy began with groups

employing frame-based immobilization techniques, but localization accuracy was extremely

limited due to difficulties reproducing patient positioning between imaging sessions and

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treatments.12 Frame-based immobilization techniques improved however, with groups fixing

patients to a frame with a vacuum pillow or body cast. These early frame-based approaches to

SBRT prompted those who practiced SBRT to develop image-based methods for target

verification.12 Frame-based SBRT methods began to include daily CT imaging for localization,

which eventually moved towards localization based on electronic portal imaging. Currently the

most common image-based methods for SBRT are kV and MV cone-beam CT (CBCT).

Stereotactic body radiation therapy requires that treatments be conducted using a fixed

3D coordinate system created by the use of fiducial markers.13 The fiducial markers used in the

treatments can vary, and may include markers on a stereotactic body frame or implanted in the

tumor itself. Any immobilization device used during treatment must coincide and reference the

stereotactic coordinate system created by the fiducial markers.13 Most importantly, the

coordinate system produced by the fiducial markers must correlate with the treatment machine

and the target lesion within the patient in a reproducible and secure manner. Due to the motion of

the lungs during the respiratory cycle, special techniques must be used to account for the effects

of the motion. Such techniques include abdominal compression, free breathing and breath hold

techniques, and respiratory gating.

Most SBRT systems are linear accelerator based. These systems incorporate either digital

x-ray or tomographic imaging to assist in localization, using CT-on-rails or on-board CBCT

imaging. Current linear accelerator based SBRT systems include units made by Varian,

BrainLab, and Elekta. Other SBRT capable systems include CyberKnife and helical

TomoTherapy. CyberKnife began in 1994 as a modernized dedicated SRS system.14 It initially

was only able to treat brain tumors, but over time technological improvements allowed treatment

of extracranial targets. The current CyberKnife system is an integrated image-guidance and

treatment delivery device that incorporates a computer controlled robotic linear accelerator with

an x-ray camera.6 This system tracks the tumor in real time during treatment, and adjusts the

position of the robotic arm to deliver the treatment. The advantage of this system is that it does

not require stereotactic body frames or complex immobilization devices.

Helical TomoTherapy involves a machine that uses a linear accelerator waveguide that

rotates in a gantry around the patient as the treatment couch moves through the gantry bore

during treatment, analogous to a CT unit.15 Helical TomoTherapy can be thought of as a helical

CT scanner equipped with a MV linear accelerator instead of a kV x-ray tube as a source of

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radiation.15 TomoTherapy is unique in that the operator no longer needs to be concerned with

parameters such as gantry angle, field size, or collimator angle. The helical TomoTherapy system

uses a ring gantry like a typical CT scanner to scan the patient and administer treatments. The

downside to this delivery method is that it acts like a helical CT scanner; it requires a “warm up”

and “cool down” before and after each treatment.15 Regardless of the technique, SBRT is an

increasingly popular treatment modality for treating lung cancer.

Statement of the Problem

The use of SBRT as a treatment method for primary and metastatic lung cancer has

shown many benefits. Some of these benefits include an extremely precise and focused

treatment, a reduced number of fractions, a highly conformal isodose distribution, and a very

rapid dose fall off, which allows healthy tissue surrounding the tumor to be spared.13 Strict

immobilization procedures and advanced imaging technologies are used in order to keep tumor

margin expansions as small as possible. Despite this, tumor motion brought on by the respiratory

cycle is still a major concern. Motion control techniques such as respiratory gating and FSB

brought on by abdominal compression may allow for a target to be treated more accurately. The

use of abdominal compression in particular can reduce excursions of the diaphragm during

breathing, theoretically reducing target motion and allowing for a target to be treated more

accurately. This study analyzed the use of SBRT with free breathing compared to SBRT with

abdominal compression to see which technique provided superior dosimetric outcomes.

Purpose of the Study

The purpose of this quantitative study was to compare 2 different methods of SBRT used

at 2 different centers; Center A, which performs SBRT while the patient breathes freely, and

Center B, which performs SBRT with abdominal compression. A total of 10 patients (5 from

each center) treated from January 2012 through December 2012 were randomly selected with the

purpose of analyzing their SBRT treatment plans. Dosimetric data from the treatment plans was

collected in the Pinnacle3 treatment planning system (TPS). The treatment plans were evaluated  

on target coverage and dose spillage metrics provided in RTOG-0236,1 such as total lung volume

without the PTV receiving 2000 cGy, the mean dose to the total lung volume without the PTV,

the maximum dose 2 cm from the PTV, the ratio of prescription isodose volume to the PTV

volume, high and low dose spillage, prescription isodose volume conformity, as well as

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treatment planning results (OR minimum dose, maximum dose, mean dose, etc.) to determine if

one method was superior to the other.

Assumptions of the Study

This study compared the treatment planning results of SBRT performed while free

breathing and with abdominal compression. The use of abdominal compression limits the

excursion of the diaphragm while breathing, and should reduce motion of tumors in lower lung

lobes. For lower lung lobe tumors, it was assumed SBRT with the use of abdominal compression

would have improved treatment planning results compared to SBRT with free breathing. For

upper lung lobe tumors, it was assumed that SBRT with the use of abdominal compression

would have similar treatment planning results to SBRT with free breathing. The overall

assumption was that SBRT with abdominal compression would have slightly improved treatment

planning results compared to SBRT with free breathing.

Definition of terms

Abdominal Compression. Abdominal compression reduces diaphragm excursion while

still permitting limited normal respiration.10 It can be applied in a number of ways, usually in

conjunction with a stereotactic body frame.

Accelerated Fractionation. Accelerated fractionation, or hypofractionation, provides

similar radiation doses to conventional fractionation but in less time.16 The total prescribed dose

is divided into larger dose fractions.

Alpha Cradle. An alpha cradle is an immobilization device and foaming agent used to

immobilize practically any anatomical part. The foaming agent is set in a protective sheet and

expands, contouring to the shape of a patient.16

Biological Effective Dose Equivalent. The biological effective dose equivalent takes

into account that different types of radiation cause different amounts of biological damage.16

Centigray (cGy). Centigray is the unit of energy absorbed per unit mass of any material.

1 cGy = 1 rad.16

Charlson Comorbidity Index. The Charlson Comorbidity Index is a test that predicts

patient prognosis based on comorbid conditions.

Clinical Target Volume (CTV). The CTV consists of the demonstrated tumor(s) if

present and any other tissue with presumed tumor.17

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Computed Tomography (CT). A CT scan is an ionizing radiation-based technique in

which x-rays interact with a highly sensitive scintillation crystal. Beams of radiation are sent

through the body and tissues absorb small amounts of the radiation. The absorption of radiation

in tissue allows for the production of images that show slices of the body. The result is a series of

scans that allows for the examination of sections a patient anatomy.16

Cone-Beam Computed Tomography (CBCT). Cone-beam computed tomography is a

form of volumetric imaging using an integrated accelerator with an x-ray system.16 It creates a

3D CT data set with the patient in treatment position.

Conformal Radiation Therapy. Conformal radiation therapy is a radiation therapy

technique that uses 3D images of the tumor so that multiple radiation beams can be shaped to

conform to the contour of the tumor volume.16

Conformity Index. A conformity index measures how well the volume of a dose

distribution conforms to the shape of a target volume.

Contouring. The act of contouring is delineating structures by outlining their anatomic

borders.16

Convolution Algorithm. A convolution algorithm is a dose calculation algorithm used in

3D planning systems. Dose to all points from primary radiation is computed, and then scattered

radiation from the primary dose depositions is added to obtain the total dose; contour

irregularities and tissue heterogeneities can be taken into account.16

Critical Structures. Critical structures comprise normal tissues whose radiation

tolerance limits the deliverable dose.16

CT-on-Rails System. In the CT-on-Rails system, the linear accelerator treatment table is

rotated 180 degrees and a CT unit moves on rails to image a patient while the table stays

stationary.16

CyberKnife. The CyberKnife system consists of a linear accelerator mounted on a

robotic manipulator and an integrated image guidance system.14 It is highly effective for SBRT

delivery.

Digitally Reconstructed Radiograph (DRR). A DRR replaces fluoroscopy and film

with CT simulation. It is an image similar in appearance to a conventional radiograph, but

digitally displayed on a video monitor.16

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Dose Rate. The dose rate is the amount of radiation exposure produced by a treatment

machine or source as specified at a reference field size and at a specified reference distance.16

Stereotactic body radiation therapy employs a high dose rate.

Dose Volume Histogram. A dose volume histogram is a plot of target or normal

structure volume as a function of dose.16

External Beam Radiation Therapy. External beam radiation therapy is the use of

external beam x-rays, electrons, protons, or gamma rays to be delivered to a tumor or lesion.16

Four Dimensional Computed Tomography (4DCT). Each image obtained through 4D

CT scan corresponds to a specific phase of the respiratory cycle at which the image was

acquired.16 The complete 4D CT data set displays the complete target motion during the

breathing cycle.16

Gross Tumor Volume (GTV). The GTV is the gross demonstrable extent and location

of the tumor.17

Image-Guided Radiation Therapy (IGRT). Image-guided radiation therapy is the use

of imaging methods such electronic portal imaging devices, in-room CT scanner, kV CBCT, MV

CBCT, or ultrasound to assist in targeting a lesion during radiation treatment.16

Immobilization Device. An immobilization device is a custom-designed body mold

fabricated to keep a patient in the same position.16

Internal Target Volume (ITV). The ITV is created by adding an internal margin to the

CTV, which takes into account variation in size, shape, position, and movement of the CTV

during treatment.17

Intensity-Modulated Radiation Therapy (IMRT). Intensity-modulated radiation

therapy can deliver non-uniform exposure across a beam’s eye view with a variety of techniques

and equipment.16 Areas of low dose in the target from one field are compensated by larger doses

delivered through another gantry angle that does not intersect the protected structure. Several of

these non-coplanar, intensity-modulated fields are produced, which results in high doses of

radiation delivered to targets that are irregularly shaped or close to critical structures. These non-

uniform exposures create even dose distribution to target volumes with steep dose gradients to

adjacent normal tissue.

Isodose Distribution. An isodose distribution is a 2D spatial representation of dose.16

17    

Linear Accelerator. A linear accelerator is a radiation therapy treatment machine that

makes use of high-frequency electromagnetic waves to accelerate charged particles to high

energies via a linear tube.16

Localization. Localization in radiation therapy refers to geometric definition of the

position and extent of a tumor or anatomic structure by reference of surface landmarks for

treatment setup.16

Maximum Intensity Projection (MIP). A MIP is a fusion of CT scans at full inhalation

and full exhalation. This scan shows the maximum amount of tumor movement during the

respiratory cycle.16

Metastases. Metastases is defined as the spread of cancer beyond the primary site.16

On-Board Imagers. An on-board imager is a portal imaging device built in to a linear

accelerator, which takes kV images that resemble conventional simulation images and

diagnostic-quality x-ray images.16

Organs at Risk (OR). An organ at risk is an organ in proximity to a tumor where

collateral damage done to the organ may put a patient at risk of serious injury or death.16

Planning Target Volume (PTV). The PTV is the contoured volume that includes the

CTV with an internal margin and setup margin for patient movement setup uncertainties.17

Portal Imaging. Portal imaging is performed at the beginning of treatment and at regular

intervals over the course of treatment to verify isocenter placement and beam position.16

Positron Emission Tomography (PET). A PET scan creates digital images of chemical

changes that take place in body tissue.16 An injection of glucose and radioactive material causes

the chemical changes.

Radiation Pneumonitis (RP). Radiation pneumonitis is the inflammation of lung tissue

caused by radiation therapy to the thorax.16

Respiratory Cycle. The respiratory cycle is the process of breathing. A healthy, resting

adult breathes in and out, one respiratory cycle, about 12 to 16 times per minute or

approximately 1 cycle every 4 seconds.16

Respiratory Gating. Respiratory gating is a method of treatment where the treatment

machine can be programmed to turn on only during specific phases of the respiratory cycle. The

respiratory cycle is tracked with an infrared camera and a marker placed on the chest or abdomen

or other methods.18 This allows the target to be treated at a specified position.

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Stereotactic Body Radiation Therapy (SBRT). Stereotactic body radiation therapy is a

radiation therapy technique treatment method that can deliver a high dose of radiation to a target,

utilizing either a single dose or a small number of fractional doses with a high degree of

precision within the body.12

Therapeutic Ratio. In radioimmunotherapy, the therapeutic ratio is a comparison of

tumor dose to the dose to the most sensitive normal tissues.6

Vac-Lok. Vac-Lok is an immobilization device that consists of a cushion and a vacuum

compression pump used to vacuum the cushion until it is rigid, molding to the shape of a

patient.16

Vx. Vx stands for the volume of lung receiving at least “x” Gy.16 For example, V20 stands

for the volume of lung receiving at least 20 Gy.

Limitations of the Study

The main limitation of this study was the difference in the amount of patients treated with

4D-SBRT with abdominal compression as opposed to free breathing. It was necessary to restrict

the abdominal compression patient sample size to accommodate the free breathing patient

sample size. There was also a problem with respect to treatment plans for the abdominal

compression patients. The two types of SBRT were performed at different centers; some of the

abdominal compression patients were treated on a linear accelerator, which was not available in

the treatment planning system at the other center. Thus it was necessary to further restrict the

abdominal compression patient sample size to patients with treatment plans compatible with

linear accelerators at both centers. A higher sample size would have provided more data and an

improved data analysis. Another limitation to this study was the various locations of tumors

inside the lungs. It was difficult, if not impossible, to compare an individual patient plan to

another, as certain organs may have received a higher dose based solely on tumor position.

Another limitation to this study was the algorithm of the Pinnacle3 TPS. Lax et al19 concluded

that this algorithm only gives a relatively accurate estimate of dose, compared to Monte Carlo

algorithms, in the lung volume outside of the GTV.

Methodology

This quantitative study retrospectively analyzed two different techniques used in 4D-

SBRT treatment of lung cancer. The first technique was to perform 4D-SBRT while the patient

breathed freely, and the second technique was to perform 4D-SBRT while the patient underwent

19    

abdominal compression to induce FSB. A total of 10 patients from January 2012 through

December 2012 were selected (5 treated with each technique) for this study. Each patient

underwent a 4DCT simulation scan. Once the scan was completed, the information was used to

construct 10 different CT scans related to specific stages of the respiratory cycle. These scans

were used to create a MIP dataset where the physician contoured an ITV. Once the ITV was

contoured, treatment planning was performed on the average of the reconstructed 4DCT dataset

in the Pinnacle3 TPS. The treatment plans were copied to a test environment in the Pinnacle3 TPS

in order to study metrics provided in the RTOG-02361 protocol such as the total lung volume

without the PTV receiving 2000 cGy, the mean dose to the total lung volume without the PTV,

the maximum dose 2 cm from the PTV, the ratio of prescription isodose volume to the PTV

volume, high and low dose spillage, prescription isodose volume conformity, and doses to OR.

20    

Chapter II: Literature Review

For both men and women, lung cancer is the leading cause of mortality among all

cancers.2 It is estimated that in 2013, more than 225,000 new cases of lung cancer and 155,000

deaths due to lung cancer are expected in the United States.3 Radiation therapy can be

particularly effective for lung cancer due to the response it evokes. Stereotactic body radiation

therapy is a relatively new and effective treatment modality and is increasingly popular for small

lung tumors. Stereotactic body radiation therapy provides an extremely precise and focused

delivery of a small number of fractions of radiation, to an ablative dose, to extracranial targets.

The treatment planning for SBRT requires a highly conformal isodose distribution with very

rapid dose fall off, which makes it possible to spare surrounding healthy tissue or structures from

collateral damage, even at an ablative dose.6 In order to accomplish this, SBRT employs strict

immobilization and advanced imaging technologies so that tumor margins of expansion can be

kept to a minimum.6

Stereotactic body radiation therapy requires advanced imaging and localization

technologies during simulation and treatment. To aid in localization and immobilization, molded

cradles, stereotactic body frames, and vacuum bag systems can be used in combination with

abdominal compression or ABC to reduce tumor motion.20 Murray et al21 sought to outline the

steps involved in the proper use of a rigid immobilization device for SBRT treatments. They

claimed that there are 4 steps in the treatment process that are essential to ensure accurate SBRT

treatments when using a stereotactic body frame. These steps included patient immobilization,

motion control of tumors and organs, treatment and planning correlation, and daily targeting with

pretreatment quality assurance. All patients in the study were simulated in a Vac-Lok bag, which

attached to a stereotactic body frame, creating a rigid structure that conformed to the shape of the

patient. Next, the patient was marked and tattooed at a point delineated by a laser positioning

device; this point correlated to a coordinate system labeled on the body frame. Once the patient

was immobilized in the frame, motion of the tumor and nearby organs was monitored under

fluoroscopy. An abdominal compression plate was then placed just below the xiphoid process of

the sternum. Once an acceptable tumor motion was observed, the abdominal compression plate

parameters were recorded for reproducibility. A CT scan was then performed. A GTV was

delineated and the physician placed the isocenter. The GTV was then expanded to a PTV

according to specific institutional guidelines. At the institution where the study was conducted,

21    

treatment planning was conducted with the Phillips Pinnacle TPS using non-coplanar beams

uniformly distributed around the patient. Once planning was completed, the isocenter position

with regard to the stereotactic body frame fiducials was identified. Before each treatment, the

patient underwent a verification CT scan and a new isocenter was placed in the center of the

GTV; the new isocenter was compared to that of the treatment planning CT. Once this was

completed, the patients were transferred to the treatment room where a number of quality

assurance double checks were performed to ensure proper patient positioning. The study

concluded that frame-based SBRT prevented serious misadministration for those utilizing a

conventional CT scanner for pretreatment imaging. While technologies such as on-board CBCT

could allow for reduced setup margins and easier daily setup, it was still recommended that

frame-based SBRT be used.

Another example of a rigid immobilization technique was presented by Zhou et al,22 who

presented their clinical implementation of an immobilization and localization system, BodyLoc,

combined with a TomoTherapy treatment unit to reduce set-up error and treatment time. For

each patient in the study, a GTV was obtained from a PET-CT scan, and a patient-specific

margin was added to create a PTV. The goal of treatment planning was to achieve 98% of the

PTV volume receiving the prescribed dose of 3000-6000 cGy in 3-5 fractions. A BodyLoc

system was used for patients for both a CT simulation and treatment. All patients underwent

coached free breathing during the planning CT scan. Using software designed for the BodyLoc

system, the isocenter was placed and verified using a fusion between a pretreatment CT and

planning CT. The study concluded that rigid immobilization devices greatly improved set-up

accuracy and reduced treatment time. However, in order to reduce set-up deviation and internal

organ motions, a real-time tumor tracking and dose delivery system would be required.

Not all SBRT treatments are performed with rigid immobilization devices. Frameless

SBRT is also a safe and effective method of SBRT.23, 24 Nath et al23 conducted a study on

frameless image-guided SBRT for the treatment of lung tumors with 4DCT and 4DPET/CT, and

determined that it is an effective treatment. Over the course of 2.5 years, 85 lung tumors (35 lung

metastases and 50 stage T1/T2 presumed NSCLC) were treated using SBRT with 3-5 fractions.23

Patients were set up using customized vacuum bags, wings boards with arm handles, and

respiratory position management blocks placed on the abdomen. Accompanying software for the

block was used to obtain 4DCT scans where the patient was instructed to breathe freely and

22    

normally. The 4DCT scans were fused with 4DPET images; these fused images were used for

target contouring. Internal target volume contouring was performed on MIP images fused with

their corresponding 4DPET images. The treatment margin from ITV to PTV was set to 5 mm

uniform for tumors in the upper lobe and 5 millimeters in all directions except for 8 mm in the

superior-inferior direction for tumors in the middle and lower lobes. All treatment plans were

created in the Eclipse TPS using 3D conformal radiation therapy or sliding-window IMRT. The

median dose prescribed was 4800 cGy in 4 fractions. Normal tissues monitored included lungs,

aorta, spinal cord, brachial plexus, esophagus, heart, and ribcage. The results of the study showed

a median overall survival for all patients of 31 months and a median local failure-free survival of

30 months. Local control at 2 years was 87%, which was within range of published studies.23

A similar study conducted by Sonke et al24 sought to quantify the localization accuracy

and intrafraction stability of lung cancer patients treated with frameless 4D-CBCT –guided

SBRT. Sixty-five patients with medically inoperable early stage lung cancer qualified for the

study. Each patient underwent a 4DCT scan in the supine position without a body frame or any

other immobilization devices while free-breathing without abdominal compression. For each

treatment, three 4D-CBCTs were acquired: before treatment to measure and correct the mean

tumor position, after the correction to validate the correction applied, and after treatment to

estimate intrafraction stability of the tumor. The study concluded that SBRT without a body

frame could be administered safely using 4D-CBCT guidance.

Regardless of the immobilization technique used, the addition of abdominal compression

is popular for SBRT of the lung. In theory, the use of abdominal compression should limit target

motion during the respiratory cycle. Heinzerling et al25 analyzed tumor motion during

stereotactic treatment with abdominal compression of lower lung and liver tumors with 4DCT

scans. In this study 10 patients underwent three 4DCT scans; one while breathing freely and two

at different levels of abdominal compression. The analysis found an average reduction of tumor

motion in the superior and inferior directions by 37.5% with medium compression force, and

49.2% with heavy compression force. Further, the average overall reduction in tumor motion was

39% with medium compression force, and 47.1% with heavy compression force. The study

concluded that abdominal compression can significantly reduce tumor motion in all directions,

but more research is needed on the dosimetric effects that reducing tumor motion with abdominal

compression has on tumor dose and dose to normal tissue.25

23    

A similar study conducted by Kontrisova et al26 sought to evaluate the dosimetric

consequences of irradiated lung tissue during different respiration conditions for

hypofractionated SBRT.26 This study compared a typical SBRT technique performed during free

breathing while the patient was immobilized using a body frame. Treatments were performed in

deep inspiration and expiration using standard and reduced margins, and during shallow

breathing via abdominal compression with individual margins. Thirteen patients undergoing

hypofractionated SBRT were included in the study. Two of the patients were treated for primary

lung lesions, while the rest were treated for lung metastases; all tumors were located in different

lobes of the lungs. The patients were immobilized with a stereotactic body frame with an

individually adapted vacuum pillow attached. Every patient underwent a planning CT scan in

treatment position breathing freely with abdominal compression. Additional multi-slice CT

studies were performed during free breathing without abdominal pressure, deep inspiration hold,

and deep expiration breath hold. All SBRT patients were treated with 3750 cGy in 3 fractions

prescribed to the 65% isodose level. Along with the SBRT treatment plan, 6 additional treatment

plans were created with free breathing, deep inhalation breath hold (2 plans with different

margins), deep expiration breath hold (2 plans with different margins), and free breathing with

abdominal compression. The study concluded that SBRT performed with a stereotactic body

frame and free breathing with abdominal compression favorably compared with deep inhalation

breath hold and deep expiration breath hold techniques.26

Research conducted by Huang et al27 sought to determine the dosimetric accuracy of

SBRT of lung cancer using MIPs and average 4DCT images. A custom built motion platform

and cubic lung phantom was used to simulate respiratory motion during SBRT.27 This setup was

able to recreate a number of superior-inferior motion patterns which included free breathing with

maximum motion ranges of 21.5 mm, 10 mm, and 9.5 mm, and abdominal compression with a

maximum motion range of 4.4 mm. Four dimensional computed tomography scans were taken of

all breathing motions with a Phillips Big Bore CT scanner. The 4DCT data sets were

reconstructed into composite MIP and average images and imported into the TPS. In the TPS,

ITVs were contoured on the MIP images and expanded by 0.5 mm to generate a PTV. Treatment

planning and dose calculations for the MIP based PTVs were performed on the average 4DCT

images. The study found that the 4DCT generated MIP images did not accurately depict the

maximum motion of the phantom with free breathing at a maximum motion range of 21.5 mm. It

24    

was found that abdominal compression could limit the maximum range of motion of a target and

could make targeting much more accurate. It was also recommended to obtain CBCT scans prior

to SBRT to ensure accurate targeting and that target motion had not deviated away from the

4DCT defined PTV.

Stereotactic body radiation therapy is a treatment modality known for a highly conformal

isodose distribution with a very rapid dose-fall off which is equally effective for primary lung

tumors as it is metastatic lung tumors. Guckenberger et al28 evaluated the outcome of treatment

following image-guided SBRT for early stage NSCLC and pulmonary metastases. The study was

retrospective and included 124 patients treated for 159 pulmonary target volumes between 1997

and 2007. Forty of these patients were treated for inoperable early-stage NSCLC, while 84 were

treated for pulmonary metastases. All patients were simulated and treated in a stereotactic body

frame or BodyFix system. All patients were treated while breathing freely, with abdominal

compression being added if tumor motion exceeded 5 mm in the superoinferior direction. A

GTV was delineated on a CT scan, expanded to a CTV without additional margins, and further

expanded to an ITV by adding the CTVs at inhalation and exhalation. A PTV was generated by

expanding the ITV by 5 mm in all dimensions. The treatment dose was prescribed to the PTV

surrounding isodose of 65% or 80%, depending on target location. The study concluded that

doses greater than 1000 cGy biologic effective dose to the CTV, based on 4D dose calculation,

resulted in exceptional local control rates for image-guided SBRT of primary early-stage NSCLC

and pulmonary metastasis.28

It has already been discussed that SBRT can be performed for both primary and

metastatic lung tumors, but SBRT is also commonly used for the treating early-stage NSCLC

regardless of the amount of primary lung tumors. Creach et al29 retrospectively analyzed the

clinical outcomes of patients with multiple primary lung cancers treated with SBRT. In this

study, patients were immobilized using either the Elekta Stereotactic Body Frame, the BodyFix

system, or an Alpha cradle.29 Abdominal compression was applied if a tumor moved more than 1

cm in any direction as shown by a 4DCT scan. Maximum intensity projection images from the

4DCT scan were registered to a helical CT scan for treatment planning. An ITV was defined on

the MIP images, which represented the complete motion of the GTV throughout the respiratory

cycle. The PTV was expanded from the ITV by adding a 0.5 cm margin in all dimensions.

Treatment plans were created using 7-11 non-coplanar, non-opposing beams. Treatment dose

25    

was prescribed to the 80% isodose line and covered more than 95% of the PTV. Typical dose

fractionation schemes included 5000 cGy in 5 fractions and 5400 cGy in 3 fractions depending

on tumor location. Patients were monitored with physical examinations and alternating chest x-

ray or CT scans every 3 months for 2 years following treatment. The overall survival and

progression-free survival rates of the population were monitored. The results of the study showed

that patients with metachronous (two nodules appearing at different times) lung tumors had high

survival rates, and local therapy was justified. However, patients with synchronous (two nodules

at the same time) tumors displayed poor survival rates despite having exceptional local control.

The study concluded that SBRT is an effective alternative local treatment to surgery for

medically inoperable patients with multiple primary lung cancers.29

A similar study conducted by Norihisa et al30 sought to retrospectively analyze SBRT

outcomes for oligometastatic lung tumors. Oligometastases refer to a small number of metastatic

lesion limited to a particular organ.30 All patients in this study met the following criteria: 1 or 2

pulmonary metastases, tumor diameter of less than 4 cm, a primary tumor that was locally

controlled, and no other metastatic sites. A total of 34 patients were treated over a period of 6

years; 25 of which had a single pulmonary tumor (the rest had two). All patients were simulated

with a combined x-ray and CT simulator in a stereotactic body frame. Treatment planning was

performed using CADPLAN and Eclipse. The ITV was delineated on the CT images and was

expanded by 5 mm in all dimensions to a PTV. Another 5 mm margin was added to the PTV,

which extended to the edge of the multileaf collimator for penumbra. A total of 5 to 7 non-

coplanar static 6-MV photon beams were used for each plan, irradiating 1200 cGy in each

fraction (4-5 fractions) at the isocenter for a total dose of 4800 cGy or 6000 cGy. The study

concluded that a 6000 cGy fractionation scheme was superior to 4800 cGy for local control at 2

years. The study further concluded that SBRT for oligometastatic lung tumors was comparable to

surgical metastasectomy with regard to the 2-year overall survival rate.30

Stereotactic body radiation therapy is an alternative to surgery for those unfit for surgery

and has generally minor side effects. Widder et al31 sought to investigate survival and local

recurrence rate after SBRT or 3D CRT administered for early stage primary lung cancer and to

investigate changes of health-related quality of life (HRQOL) parameters after either treatment.31

Over the course of 3 years, 202 consecutive patients with medically inoperable early stage

primary lung tumors were treated with SBRT. The treatment results were compared to a control

26    

group of patients treated with 3D CRT. This control group exhibited characteristics similar to

those treated with SBRT, which included: medically inoperable early stage primary lung cancer

with a maximum tumor diameter of 5 cm and a World Health Organization Score (WHO) of 0 to

2. All patients treated with SBRT were positioned in a vacuum-mattress and underwent a 4DCT

scan. The ITV was derived by delineating the visible GTV on a MIP reconstructed from 4-6

respiratory phases. A PTV was created by adding a 5 mm margin in all dimensions. Three

different fractionation schedules were used and were determined by tumor location in the lung.

All patients received a total dose of 6000 cGy, which was prescribed at the margin of the PTV,

constituting 80% of the dose at the isocenter. Treatments were delivered using 4 non-coplanar

dynamic arcs. The results of the study showed improved HRQOL and overall survival rates for

SBRT over 3D CRT. The study concluded that SBRT should be preferred over conventional

radiotherapy for inoperable patients with early stage NSCLC.31

Stereotactic body radiation therapy may prove to be an effective treatment for lung

cancers, but high dose fractionations used in SBRT can cause radiation-induced toxicities. The

chance of radiation induced toxicities increases when the target volume is located close to an

OR.32 A study conducted by Song et al33assessed the results and toxicity of body frame SBRT

for medically inoperable early stage lung cancer adjacent to the central large bronchus. The

results were compared with survival rates and SBRT-related toxicities of SBRT in peripheral

lung tumors.33 All patients within the study met the eligibility criteria, meaning: all were

diagnosed with NSCLC, confirmed to be stage 1 according to the American Joint Committee on

Cancer, had tumor sizes that were less than 5 cm in longest diameter, and Eastern Cooperative

Oncology Group performance status 2 or below. The technique used for the SBRT treatments

included a vacuum-fitted stereotactic body frame. Tumor motion was assessed fluoroscopically.

Tumor motion was minimized by ABC, abdominal compression, or respiratory gating. All

patients in the study underwent a CT simulation with contrast enhancement. A CTV was

delineated on axial CT images and was expanded to a PTV by adding a 5 mm margin to the axial

plane and a 10 mm margin to the longitudinal direction of the CTV. Daily setup accuracy and

tumor motion were checked via on-board CBCT. The treatment dose was prescribed such that

85% of the isodose would cover 95% of the PTV volume. Stereotactic body radiation therapy

was performed in 3 or 4 fractions, with doses of 1000, 1200, or 2000 cGy. Following treatment,

96.9% of the patients experienced pulmonary toxicities, with most patients showing only mild

27    

symptoms. Three patients with centrally located tumors had severe pulmonary toxicities, and 8

developed partial or complete bronchial stenosis. One patient died of bleeding, aspiration, and

pneumonia from SBRT-induced complete bronchial stenosis. The study concluded that SBRT is

an effective treatment modality for patients with inoperable early stage NSCLC, but SBRT

should not be given in high dose fractionation to centrally located tumors because it can cause

substantial airway toxicities.33

Another possible SBRT induced toxicity is radiation pneumonitis (RP). Research has

been conducted concerning risk factors for RP following SBRT for NSCLC.31,32,33,34 A study

conducted by Barriger et al34 discussed the frequency and correlation for RP after SBRT for

NSCLC. Two hundred fifty-one SBRT plans from 2000 to 2008 were reviewed for patients with

early stage NSCLC who had no prior chest radiation therapy.34 The treatment plans incorporated

5 to 12 non-coplanar beams and delivered 2400-7200 cGy in 3-5 fractions. The results of the

study showed that the overall rate of RP in the sample was 9.4%. Development of symptomatic

RP correlated significantly with MLD and V20 but not with V5, V10, PTV volume, or tumor

location. The study concluded that with modern treatment planning advances and increased

planner experience, MLD can be kept to a minimum.34

Matsuo et al35 also sought to identify any dose-volume factors that could be associated

with radiation RP after SBRT for lung cancer. All patients in the study were diagnosed with

inoperable stage I NSCLC based on CT studies. For this study, the patients were immobilized in

a stereotactic body frame and treated with non-coplanar 6 MV x-ray beams from a Clinac 2300

linear accelerator. Stereotactic body radiation therapy was planned on the Eclipse TPS, where an

ITV was delineated on slow-scan CT images, with tumor motion being assessed by x-ray

fluoroscopy. The ITV was expanded to a PTV by adding a 5 mm margin for setup uncertainty. In

this study, the following dose-volume metrics were evaluated: PTV volume (ml), MLD, V5, V10,

V15, V20, V25, V30, V35, and V40, where Vd is the volume of normal lung tissue that received more

than a dose of d Gy. Of the 74 patients in the study, 15 developed symptomatic RP. The results

of the study showed that PTV volume and V25 were the most significant factors that attributed to

symptomatic RP; as PTV volume and V25 increased, the incidence of symptomatic RP increased.

It should be noted that the study only indicated that a large PTV is a significant risk factor for

symptomatic RP after SBRT, and does not indicate other factors such as tumor location and

previous radiation therapy to the lung.

28    

Baker et al36 also conducted research on dosimetric predictors of RP. In this study,

dosimetric information was taken from 263 SBRT plans of 240 patients which included MLD,

V5, V13, V20, Vprescription, and a conformity index defined as the prescription isodose line divided

by the volume enclosed by the PTV.36 Other information analyzed included the size of the GTV

(in cm3) and PTV, as well as the size of the total lung volume without the GTV. Of the 263

patient plans analyzed, 230 of them involved the use of abdominal compression in conjunction

with a BodyFix vacuum cradle for immobilization. Of the 240 patients, 26 developed grade 2

pneumonitis, and 3 developed grade 3 pneumonitis. The study concluded that female gender,

number of pack-years smoking, and the Charlson Comorbidity Index were significant predictors

of RP. Significant dosimetric predictors of RP included V5, V13, V20, Vprescription, and the ratio of

the PTV/total lung volume without the GTV.36

A study conducted by Guckenberger et al37 sought to evaluate tumor response and

pulmonary injury from CT morphological patterns after SBRT for early stage NSCLC and

pulmonary metastasis. A retrospective analysis was conducted on a total of 70 patients who were

treated with SBRT for pulmonary metastases or for inoperable primary early stage NSCLC over

a period of 7 years.37 The patients were immobilized in a stereotactic body frame with abdominal

compression being applied if fluoroscopy showed that breathing induced motion of the

pulmonary tumor more than 5 mm. The GTV was delineated on the CT scan, with no additional

margin added for the generation of the CTV. An ITV was created by including tumor positions

in inspiration and expiration. The PTV was created by adding a margin of 5 mm in all

dimensions to the ITV; this margin compensated for error in tumor position and changes from

breathing motion during treatment. Treatment techniques included 3D conformal multi-fields

with 5 to 7 coplanar and non-coplanar beams. The treatment dose was prescribed to the PTV-

enclosing isodose line and was normalized to 100% of the maximum dose. Fractionation

schemes differed depending on tumor size and location. A CT simulation was performed prior to

every treatment to verify the position of the isocenter relative to the target. The study concluded

that SBRT for primary early stage NSCLC resulted in high local control rates with low

pulmonary toxicity rates. The study recommended that patients be closely monitored for

pneumonitis up to a year following treatment, as it could be misdiagnosed as infectious

pneumonia.37

29    

Takayama et al38 sought to analyze their own SBRT treatment planning procedures and

their results in terms of doses to the ITV and OR using dose-volume histograms (DVHs). Every

patient was immobilized in the supine position with a stereotactic body frame along with a

vacuum pillow.38 Tumor motion was monitored via fluoroscopy during free breathing; if the

tumor moved more than 10 mm in the craniocaudal direction, abdominal compression was

applied before at CT scan was performed. Only an ITV was delineated by the physician due to

the CT scan including internal motion. The physician also contoured the lung, esophagus, spinal

cord (canal), heart, and pulmonary artery as OR. Treatment planning was performed with 5-10

non-coplanar 6MV static beams. A treatment dose of 4800 cGy in 4 fractions was prescribed at

the isocenter, with a goal of maintaining a range of dose to the ITV within 90% to 110% of the

isocenter dose. Another goal was to maintain the volume of irradiated bilateral lung with 2000

cGy or more to less than 25%. The results of the study confirmed that their SBRT treatment

technique provided homogenous dose distribution to the target while sparing OR. The only

reported normal tissue toxicities were Grade 1 and 2 RP, with Grade 1 accounting for 92% of all

pulmonary toxicities. These patients were all asymptomatic and only demonstrated pneumonitis

on follow up CT scans. The study concluded that the use of multiple non-coplanar static ports in

SBRT achieve homogenous dose distribution while avoiding high dose to normal tissues.38

Radiation pneumonitis is but one radiation induced toxicity that can be caused by SBRT.

However, there can be side effects of SBRT that aren’t necessarily harmful. A study by Martin et

al39 was performed to determine the amount of incidental dose given to the mediastinal and hilar

nodal regions for patients with NSCLC treated with SBRT to a dose of 6000 cGy in 3 fractions.

This dose and fractionation was chosen because it is commonly used. The treatment using SBRT

was delivered according to guidelines set by the RTOG-02361 protocol. The GTV was outlined

on a free breathing CT scan and expanded to a PTV by 5 mm in the axial direction and 10 mm

superior and inferior. A vacuum bag and abdominal compression device were used to limit

respiratory motion. A 4DCT scan was used to define the ITV. A minimum of 9 non-opposing,

non-coplanar, 6MV photon beams were set up in the Eclipse TPS to treat the PTV. The study

concluded that the incidental irradiation delivered during SBRT for lower lobe NSCLC can be

therapeutic for low-volume, subclinical nodal disease. Treating lung tumors of the upper lobe has

a lower likelihood of therapeutic benefit for the nodes.39

30    

Stereotactic body radiation therapy is a relatively new treatment modality, and much

work needs to be done to improve the safety and practice of it. A study conducted by Perks et

al40 sought to improve the quality and safety of their practice of SBRT by analyzing the overall

treatment process and performing a failure mode and effect analysis (FMEA). A FMEA focuses

on the individual steps of a process, a mode of failure for a process and possible causes.40 Three

values are derived for any mode of failure and its causes: the probability of it occurring, the

severity of the consequences if the failure is not recognized, and the ability to catch the failure. A

value of 1 to 10 is given to each quantity of probability, severity, and detectability, then these

values are multiplied by each other to determine a risk priority number (RPN). Whichever step in

the process has the highest RPN would then be targeted for improvement to patient safety.40 In

this study, treatment steps with the highest RPN scores included CBCT isocenter misalignment,

anatomic setup failure, improper laser markings or transcription error for couch movements, and

patient movements during treatment. For all of these treatment steps, the study suggested the

need for rigorous quality assurance.40

31    

Chapter III: Methodology

Stereotactic body radiation therapy is a treatment for primary and metastatic lung tumors

that is growing in popularity. Stereotactic body radiation therapy offers a precise and focused

delivery of a small number of fractions of radiation to an extracranial target such that a highly

conformal isodose distribution with a rapid dose fall-off is created.7 This type of therapy makes it

possible to deliver a high dose of radiation to a target while sparing any surrounding healthy

normal tissue. Despite this, tumor motion brought on by the respiratory cycle is still a major

concern. Motion control techniques such as respiratory gating and FSB brought on by abdominal

compression allow for a target to be treated precisely and accurately.

In this quantitative study, Center A employed SBRT for the treatment of small lung

tumors; these treatments were performed while the patient breathed freely. This method allowed

the patient to breathe comfortably during treatment. Center B also employed SBRT for the

treatment of small lung tumors, but the treatment was performed while the patient was forced to

breathe shallow through abdominal compression. The purpose of the study was to determine

which method was dosimetrically superior. The importance of sample selection, instrumentation,

data collection, data analysis, and limitations were discussed in this chapter.

Sample Selection and Description

The patient population for this retrospective study was randomly selected. At total of 10

patients were chosen; 5 from Center A and 5 from Center B. All patients were treated at their

respective clinics between January 2012 and December 2012. The patient population was

selected from a list of cases representing both lungs and all lobes of the lungs. An attempt was

made to select the patient population from tumors of the lower lobe, but this would have severely

reduced the sample size. Patients treated at Center A were treated while freely breathing and

patients treated at Center B were treated under the effects of abdominal compression.

Instrumentation

A GE Lightspeed CT scanner equipped with GE Advantage 4D software, along with an

infrared camera and laser positioning system, was used to obtain 4DCT scans of the patients. A

Vac-Lok bag was used to simulate and treat patients at Center A, while a stereotactic body frame

was used in conjunction with an abdominal compression belt at Center B. Once the 4DCT scan

was completed, it was transferred to an Advantage 4D workstation where the information was

used to construct 10 different CT scans related to specific stages of the respiratory cycle. These

32    

scans were used to create a MIP dataset where the physician contoured an ITV. Once the ITV

was contoured, treatment planning was performed on the average of the reconstructed 4DCT

dataset. The Pinnacle3 TPS and the collapsed cone convolution superposition (CCCS) algorithm

were used for all treatment plans and dose calculations in this study.

Data Collection

Due to the retrospective nature of this study, all patients had completed their respective

SBRT treatments. The ITV and PTV for each patient had been contoured previously by the

physician, as well as critical structures. In order to analyze PTV coverage and dose spillage, it

was required to transfer all treatment plans to a test environment in the TPS. This measure was

taken to avoid compromising the integrity of the original plans. Once the plans were transferred

to a test environment, contours were created representing volumes of dose as recommended by

RTOG-0236.1

Data Analysis

Once data was collected from the treatment plans, an analysis was performed on dose

guidelines recommended in the RTOG-0236 protocol.1 These included the cumulative volume of

tissue outside of the PTV receiving a dose greater than 105% of the prescribed dose, the ratio of

prescription isodose volume to the PTV, the ratio of 50% prescription isodose volume to the

PTV, maximum dose 2 cm from PTV in any direction, and the percent of lung receiving 2000

cGy total or more.1 An analysis was then performed on OR such as whole lung, spinal cord,

esophagus, and heart. This analysis included a comparison of minimum, maximum, and mean

dose for the OR via a DVH.

Limitations

The main limitation of this study was the difference in the amount of patients treated with

4D-SBRT with abdominal compression as opposed to free breathing. It was necessary to restrict

the abdominal compression patient sample size to accommodate the free breathing patient

sample size. There was also a problem with respect to treatment plans for the abdominal

compression patients. The two types of SBRT were performed at different centers; some of the

abdominal compression patients were treated on a linear accelerator that was not available in the

treatment planning system at the other center. Thus it was necessary to further restrict the

abdominal compression patient sample size to patients with treatment plans compatible with

linear accelerators at both centers. A higher sample size would have provided more data and an

33    

improved data analysis. Another limitation to this study was the various locations of tumors

inside the lungs. It was difficult, if not impossible, to compare an individual patient plan to

another, as certain organs may have received a higher dose based solely on tumor position.

Another limitation to this study was the algorithm of the Pinnacle3 TPS. Lax et al19 concluded

that this algorithm only gives a relatively accurate estimate of dose, compared to Monte Carlo

algorithms, in the lung volume outside of the GTV.

Summary

For this retrospective study, 10 patients treated for lung cancer were selected to analyze

two different methods of SBRT. These methods included SBRT while breathing freely and

SBRT while undergoing abdominal compression to force shallow breathing. The treatment plans

were analyzed to determine their results with reference to criteria set by the RTOG-0236

protocol.1 This included the cumulative volume of tissue outside of the PTV receiving a dose

greater than 105% of the prescribed dose, the ratio of prescription isodose volume to the PTV,

the ratio of 50% prescription isodose volume to the PTV, maximum dose 2 cm from the PTV in

any direction, and percent of lung receiving 2000 cGy total or more. The data obtained was to

determine which method of SBRT is dosimetrically superior.

34    

Chapter IV: Results

Stereotactic body radiation therapy is a treatment for primary and metastatic lung tumors

that is growing in popularity. Stereotactic body radiation therapy offers a precise and focused

delivery of a small number of fractions of radiation to an extracranial target such that a highly

conformal isodose distribution with a rapid dose fall-off is created.7 This type of therapy makes it

possible to deliver a high dose of radiation to a target while sparing any surrounding healthy

normal tissue. Despite this, tumor motion brought on by the respiratory cycle is still a major

concern. Motion control techniques such as respiratory gating and forced shallow breathing

brought on by abdominal compression allow for a target to be treated precisely and accurately.

The purpose of this research was to determine if 4D-SBRT performed with abdominal

compression is dosimetrically superior to 4D-SBRT performed with free breathing. An analysis

was performed on treatment plans for 5 patients treated with abdominal compression and 5

patients treated while breathing freely. Metrics provided in the RTOG – 0236 protocol1 were

used to evaluate prescription isodose surface coverage of the PTV, high and low dose spillage,

and doses to OR. The results of this study demonstrated the effect abdominal compression had

on treatment planning results.

Item Analysis

The treatment plans of 10 patients were evaluated; patients A through E represented those

treated with 4D-SBRT while free breathing and patients F through J represented those treated

with 4D-SBRT with abdominal compression. Patients A – E represented 3 right lung tums (2

upper lobe and 1 lower lobe) and 2 left lung tumors (1 middle lobe and 1 lower lobe). Patients F

– J represented 3 right lung tumors (2 upper lobe and 1 middle lobe) and 2 left lung tumors (1

upper lobe and 1 lower lobe). The 10 treatment plans were analyzed using metrics provided in

the RTOG – 0236 protocol.1 Metrics used included total lung volume without the PTV receiving

20 Gy, total lung without PTV mean dose, maximum dose 2 cm away from the PTV, the ratio of

the prescription dose volume to the PTV volume, high and low dose spillage, prescription

isodose surface coverage, and the minimum, maximum, and mean dose to OR.1

The average total lung volume without the PTV receiving 2000 cGy or more while free

breathing was 4.5%. The average total lung volume without the PTV receiving 2000 cGy or

more with abdominal compression was 4.8%. The free breathing patients had a smaller total lung

volume without the PTV receiving 2000 cGy or more. Table 1 lists the values of the total lung

35    

volume without the PTV receiving 2000 cGy or more. As a percentage of the prescription dose,

the mean dose to the total lung volume without the PTV for patients A – E was 4.3%, 5.5%,

7.2%, 9.3%, and 4.9% respectively. For patients F – J the values were 7.8%, 3.7%, 3.1 %, 9.2%,

and 10.1% respectively. As a percentage of the prescription dose, the average mean dose to the

total lung volume without the PTV for patients A – E was 6.2%, and 6.8% for patients F – J. The

free breathing patients received a smaller average mean dose to the total lung volume without the

PTV. Table 2 lists the mean dose to the total lung volume without the PTV for each patient.

The maximum dose 2 cm away from the PTV for patients A – E was 3412.8 cGy, 2163.9

cGy, 3812.7 cGy, 3923.7 cGy, and 2486.0 cGy respectively. As a percentage of the respective

prescription dose, the maximum dose 2 cm away from the PTV for patients A – E was 68.3%,

54.1%, 76.3%, 78.5%, and 51.8% respectively. The maximum dose 2 cm away from the PTV for

patients F – J was 3269.2 cGy, 2660.1 cGy, 3310.7 cGy, 3562.5 cGy, and 4710.1 cGy

respectively. As a percentage of the prescription dose, the maximum dose 2 cm away from the

PTV was 60.5%, 49.3%, 61.3%, 71.3%, and 87.2% respectively. As a percentage of the

prescription dose, the maximum dose 2 cm away from the PTV for patients A – E averaged

65.8%, while patients F – J averaged 65.9%. The average maximum dose 2 cm away from PTV

for both sets of patients was practically the same. Table 13 lists the maximum dose 2 cm away

from the PTV for each patient.

The ratio of the prescription isodose volume to the PTV volume for patients A – E was

0.609, 0.799, 0.797, 0.931, and 0.879 respectively. The ratio of prescription isodose volume to

the PTV volume for patients F – J was 1.100, 0.958, 1.017, 1.129, and 1.126 respectively. The

average ratio of the prescription isodose volume to the PTV volume for free breathing patients

was 0.803 and 1.066 for patients with abdominal compression. This means that on average, the

prescription isodose volume covered more of the PTV volume for abdominal compression

patients. Table 14 lists the ratio of the prescription isodose volume to the PTV for each patient.

The low dose spillage, or the ratio of the 50% isodose volume to the PTV volume for

patients A – E was 4.775, 3.313, 3.714, 4.541, and 4.478 respectively. The same ratio for patients

F – J was 5.380, 4.26, 6.532, 4.496, and 4.409 respectively. Table 15 lists the low dose spillage

for each patient. The average ratio for patients A – E was 4.164 and 5.015 for patients F – J. The

average low dose spillage was lower for free breathing patients than abdominal compression

patients. The high dose spillage or the ratio of the 105% isodose volume to the PTV volume for

36    

patients A – E was 0.372, 0.617, 0.587, 0.766, and 0.569 respectively. The same ratio for patients

F – J was 0.543, 0.574, 0.549, 0.808, and 0.904 respectively. Table 16 lists the low dose spillage

for each patient. The average high dose spillage for patients A – E was 0.583 and 0.676 for

patients F – J. This ratio means that no tissue outside of the PTV received more than 105% of the

prescription isodose for both sets of patients. However, this means that the PTVs of abdominal

compression patients had larger hot spots.

To determine prescription isodose surface coverage, it needed to be determined whether

99% of the PTV received a minimum of 90% of the prescription dose. The ratio of 90% of the

prescription isodose volume to 99% of the PTV volume for patients A – E was 1.098, 1.083,

1.153, 1.202, and 1.260 respectively. The same ratio for patients F – J was 1.764, 1.432, 1.684,

1.587, and 1.507 respectively. Table 17 lists the ratio of 90% of the prescription isodose volume

to 99% of the PTV volume for each patient. The average ratio for patients A – E was 1.159 and

1.595 for patients F – J. This means that both sets of patients met the required prescription

isodose surface coverage, but 99% of the PTV volume for abdominal compression patients

received a higher minimum dose.

Finally, the minimum, maximum, and mean doses to the heart, esophagus, and spinal

cord were analyzed for both sets of patients. Due to various locations of tumors within both

lungs, it was difficult to compare treatment planning results for each set of patients. Therefore,

the results were compared to the dose limits provided in the RTOG – 0236 protocol.1 The

maximum dose to any point in the heart, esophagus, and spine met RTOG – 02361 dose limits,

with the exception of patient D. Patient D received a maximum dose of 3675.1 cGy to the heart,

which exceeds the maximum of 30 Gy. It should be noted patient D’s PTV was located in the left

lung in close proximity to the heart.

This retrospective study displayed almost identical results for both patient sets with

regard to MLD, total lung volume without the PTV receiving 2000 cGy, and maximum dose 2

cm away from the PTV. Stereotactic body radiation therapy with free breathing and abdominal

compression showed promising results, but neither outperformed the other. Stereotactic body

radiation therapy with abdominal compression had slightly better results with regard to

prescription isodose surface conformity; however, this may have been due to a higher average

prescription dose chosen for SBRT with abdominal compression. Of the free breathing patients,

3 received 5000 cGy in 5 fractions, 1 received 4800 cGy in 4 fractions, and 1 received 4000 cGy

37    

in 4 fractions. Of the abdominal compression patients, 4 received 5400 cGy in 3 fractions, and 1

received 5000 cGy in 5 fractions. Stereotactic body radiation therapy while free breathing had

better results with regard to low dose spillage. This also may have been due to a higher average

prescription dose chosen for abdominal compression SBRT patients.

38    

Chapter V: Discussion

Stereotactic Body Radiation Therapy is an effective treatment for small lung lesions.

Performing a 4DCT scan allows physicians to accurately define a target volume that represents

the complete motion of a tumor during the respiratory cycle. This in turn allows SBRT to be

performed while a patient breathes freely. To try and reduce tumor motion, abdominal

compression devices have been developed to induce forced shallow breathing. In theory,

inducing forced shallow breathing should reduce target volume and spare healthy lung tissue.

Abdominal compression has been shown to significantly reduce excursions of the diaphragm and

therefore reduce motion of tumors in close proximity.11 However, the effects of abdominal

compression are significantly reduced for upper lobe tumors.11

This study analyzed the treatment planning results of 5 patients treated with SBRT while

breathing freely and 5 patients treated with SBRT with abdominal compression. The analysis

included measuring the total lung volume without the PTV receiving 2000 cGy, the mean dose to

the total lung volume without the PTV, the maximum dose 2 cm from the PTV, the ratio of

prescription isodose volume to the PTV volume, high and low dose spillage, prescription isodose

volume conformity, and doses to OR. The results showed no significant difference in total lung

volume without the PTV receiving more than 2000 cGy, mean dose to the total lung volume, and

maximum dose 2 cm away from the PTV. Abdominal compression showed slightly better

prescription dose conformity. Free breathing showed slightly better low dose spillage. It was

uncertain how a difference in prescription dose had an effect on the results of the study.

Of the abdominal compression patients, 3 had upper lobe tumors. Bouilhol et al11 made

the conclusion that there are minimal effects of abdominal compression when used on tumors of

the upper lobe. If every patient in this study had a lower lobe tumor where tumor motion was

greatly affected by excursion of the diaphragm, it is very possible that abdominal compression

would have shown better results. Contrary to the findings of Kontrisova et al26, this study did not

find a significant dosimetric difference between the free breathing plans and abdominal

compression plans. However, this study had similar findings to Heinzerling et al25, which found

that abdominal compression for lower lobe lung tumors could significantly reduce motion, but

the effects it had on tumor dose and dose to normal tissue were unclear. A number of studies

used in this research studied radiation induced toxicities and their dosimetric predictors.31,32,33,34

None of the patients in this study reported any potentially serious radiation induced pulmonary

39    

toxicities in their 2 month follow up. However, of the 10 patients used in the study, 3 died within

a year due to non-radiation related causes.

Limitations

The main limitation of this study was the difference in the amount of patients treated with

4D-SBRT with abdominal compression as opposed to free breathing. It was necessary to restrict

the abdominal compression patient sample size to accommodate the free breathing patient

sample size. There was also a problem with respect to treatment plans for the abdominal

compression patients. The two types of SBRT were performed at different centers; some of the

abdominal compression patients were treated on a linear accelerator that was not available in the

treatment planning system at the other center. Thus it was necessary to further restrict the

abdominal compression patient sample size to patients with treatment plans compatible with

linear accelerators at both centers. A higher sample size would have provided more data and an

improved data analysis. Another limitation to this study was the various locations of tumors

inside the lungs. It was difficult, if not impossible, to compare an individual patient plan to

another, as certain organs may have received a higher dose based solely on tumor position and

differences in prescription dose. Another limitation to this study was the algorithm of the

Pinnacle3 TPS. Lax et al19 concluded that this algorithm only gives a relatively accurate estimate

of dose, compared to Monte Carlo algorithms, in the lung volume outside of the GTV.

Conclusions

The study concluded that when planned on an average 4DCT data set, SBRT performed

with free breathing and SBRT performed with abdominal compression are effective methods for

treating appropriately sized lung tumors. However, neither method was able to outperform the

other. Both methods showed similar results with regard to mean dose to the whole lung volume

without the PTV, total lung volume without the PTV receiving 2000 cGy, and the maximum

dose 2 cm away from the PTV in any direction. Patients treated with abdominal compression

showed slightly better prescription isodose prescription conformity, but it is uncertain if this is

due to the patients breathing freely receiving a slightly lower average prescription dose. Patient

treated while breathing freely showed signs of slightly better low dose spillage, but it is uncertain

how the result is affected by a difference in average prescription dose. Given the results of the

study, it was concluded that if a patient is made uncomfortable by abdominal compression,

undergoing SBRT while breathing freely is a safe and effective alternative.

40    

Recommendations

The research study was limited to only 10 patients. A much larger sample size would be

needed for future research on SBRT with free breathing and abdominal compression. It might

also help to refine any future studies with regard to tumor location. Restricting studies to a single

lung and lobe would provide more accurate results. It is also recommended that any future

studies use a constant prescription dose for the patient population. This, along with a single

tumor location, would make a direct comparison of results possible.

41    

Tables

Table 1. Percent volume of Total Lung without PTV receiving 2000 cGy

Free Breathing Abdominal Compression

A 3% F 5.5%

B 2% G 5

C 4.5% H 3%

D 7.5% I 3%

E 5.5% J 7.5%

Table 2. Total Lung without PTV Mean Dose

Free Breathing Abdominal Compression

A 214.7 cGy F 425.7 cGy

B 220.7 cGy G 197.1 cGy

C 363.1 cGy H 168.4 cGy

D 467.6 cGy I 463.2 cGy

E 234.4 cGy J 546.2 cGy

Table 3. Patient A treatment plan results

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.0 4985.6 214.7

Spinal Cord 1.6 749.7 107.2

Esophagus 5.3 673.6 265.4

Heart 0.1 15.6 4.1

42    

Table 4. Patient B treatment plan results

Table 5. Patient C treatment plan results

Table 6. Patient D treatment plan results

Table 7. Patient E treatment plan results

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.4 4158.6 220.7

Spinal Cord 4.1 1240.6 143.6

Esophagus 3.7 1169.6 301.2

Heart 15.9 1649.7 236.7

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.5 5188.5 363.1

Spinal Cord 2.6 59.5 13.5

Esophagus 4.1 1154.7 165.7

Heart 2.0 1124.5 41.2

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 1.9 5455.7 467.5

Spinal Cord 2.4 2350.9 315.6

Esophagus 12.2 1711.3 496.2

Heart 44.5 3675.1 566.5

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 1.6 4297.4 434.4

Spinal Cord 19.1 2149.0 325.4

Esophagus 3.1 1649.9 236.6

Heart 0.5 82.2 12.3

43    

Table 8. Patient F treatment plan results

Table 9. Patient G treatment plan results

Table 10. Patient H treatment plan results

Table 11. Patient I treatment plan results

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.5 5643.9 425.7

Spinal Cord 1.4 1076.0 26.5

Esophagus 7.2 174.0 24.9

Heart 1.4 1457.9 20.5

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.0 5606.6 197.1

Spinal Cord 2.9 1007.1 26.5

Esophagus 4.6 1051.6 113.3

Heart 0.2 36.3 9.4

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 0.0 5845.7 168.4

Spinal Cord 1.5 769.1 33.3

Esophagus 0.3 1793.1 176.9

Heart 0.4 26.1 6.0

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 1.7 5248.1 463.2

Spinal Cord 1.3 1519.7 247.9

Esophagus 7.9 1336.9 270.6

Heart 27.1 2045.6 467.9

44    

Table 12. Patient J treatment plan results

Table 13. Max dose 2 centimeters away from PTV (D2cm)

Free Breathing Abdominal Compression

A 3414.8 cGy F 3269.2 cGy

B 2163.9 cGy G 2660.1 cGy

C 3812.7 cGy H 3310.7 cGy

D 3923.7 cGy I 3562.5 cGy

E 2486.0 cGy J 4710.1 cGy

Average 3160.2 cGy Average 3502.5 cGy

Table 14. Ratio of prescription isodose volume to the PTV (prescription isodose volume/PTV

volume) – A value of 1 means the prescription isodose volume equals the PTV volume

Free Breathing Abdominal Compression

A 0.609 F 1.100

B 0.799 G 0.958

C 0.797 H 1.017

D 0.931 I 1.129

E 0.879 J 1.126

Average 0.803 Average 1.066

Structure Min Dose (cGy) Max Dose (cGy) Mean Dose (cGy)

Total Lung-PTV 1.3 6057.2 546.2

Spinal Cord 3.5 547.0 80.2

Esophagus 22.2 577.2 225.3

Heart 17.4 2162.6 266.4

45    

Table 15. Low Dose Spillage - Ratio of 50% isodose volume to the PTV (50% isodose

volume/PTV volume) - A higher value means more tissue outside of the PTV received 50% of

the prescription dose

Free Breathing Abdominal Compression

A 4.775 F 5.379

B 3.312 G 4.246

C 3.714 H 6.532

D 4.541 I 4.496

E 4.478 J 4.409

Average 4.164 Average 5.0124

Table 16. High Dose Spillage - Ratio of 105% isodose volume to the PTV (105% isodose

volume/PTV volume) - A value greater than 1 means an area outside of the PTV received 105%

of the prescription dose

Free Breathing Abdominal Compression

A 0.372 F 0.543

B 0.617 G 0.573

C 0.586 H 0.549

D 0.766 I 0.808

E 0.569 J 0.904

Average 0.582 Average 0.675

46    

Table 17. Prescription Isodose Surface Coverage (90% prescription isodose volume/99% PTV

volume) - A value greater than 1 means 99% of PTV is covered by a minimum of 90%

prescription dose

Free Breathing Abdominal Compression

A 1.098 F 1.764

B 1.083 G 1.432

C 1.153 H 1.684

D 1.201 I 1.587

E 1.261 J 1.507

Average 1.159 Average 1.595

47    

Figures

Figure 1. Ratio of prescription isodose volume to PTV volume

48    

Figure 2. High Dose Spillage

49    

Figure 3. Low Dose Spillage

50    

Figure 4. Prescription Isodose Surface Coverage

51    

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