patient positioning - scott memorial...

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Patient Set-ups and Tumor Localizations

Amy S. Harrison

Patient Positioning

• Prior to starting any localization or

simulation procedure patients need to

be positioned and immobilized

• Patients disease location and

physical limitations must be taken

into consideration

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Patient Alignment

Reproducible positioning of the entire

patient, not just the treatment region

is imperative

A small angle change of the patient on

the table can represent a significant

change in the delivered treatment

Exaggerated Patient Position Shift

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Patient Positioning-Brain/H+N

• Immobilized by aquaplast masks over

the head alone/ or head and shoulders

• The need for a bite block should be

addressed

• Head holders should be selected for

patient comfort and extension of neck

• Indexing of immobilization improves

reproducibility of set up

Head Immobilization

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Head Immobilization

Full Set Up Photo H+N Case

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Lung

• Alpha cradle

• Arms up? Arms down?

• ABC device

• Abdominal Compression Device

• Leg immobilization-rubber band, plastic

foot holder, angle sponge?

• Will it clear CT/MRI bore

Stereotactic Lung

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Lung

Lung

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Breast Immobilization

• Alpha cradle or wing board

• Opposite arm immobilized how?

• ABC device

• Leg immobilization-rubber band,

plastic foot holder, angle sponge?

• Will it clear CT/MRI bore

Breast

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Breast

Breast

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Breast

Prostate/Pelvis

• Pelvis-full aquaplast, alpha cradle

or nothing

• Legs on angle sponge or flat

• Feet rubber bands or foot holder

• Arms?

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Prostate/Pelvis

Prostate/Pelvis

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Prone Rectum

• Belly board, angle sponge?

• Feet?

• Patient on pillow or not?

Rectum/Prone

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Prone Rectum

Prone Rectum

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Immobilization vs. High Tech

• IGRT- cone beam or fiducials

• Tomotherapy

• Cyber Knife

• ExacTrac

Localization

Procedure where the target and critical

structures are delineated with reference to

the patient’s external surface

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Classical Localization

• The patient’s external surface was attained by

using solder wire or plaster of paris

• This surface was then drawn on a piece of

paper

• Or pantograph

2D Contours

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Classical Localization

• Tumor and critical structures were

transcribed from hard copy CT studies or MD

demarcation on the orthogonal films taken at

the time of localization

2D Contours

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2D Contours

2D Contours

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2D Contours and Coordinates

Simulation

• A procedure where the planned

fields are verified by shooting

diagnostic quality films in the

simulator (a machine which mimics

the geometry of the treatment unit))

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The CT Simulator

• The advent of the CTsim dramatically

modified the simulation and localization

procedures.

• Localization could now be done at the

time of Ctsim

• Target delineation was truly 3D

• Precision and accuracy greatly improved

An integral functionality of the CT Simulator unit is the capability of placing reference marks on the patient to

indicate the isocenter for the treatment fields.

1) Reference marks can be placed near the isocenter of

the patient. This is an estimate of the final isocenter and

does not require extensive contouring. The isocenter is

found through a series of exact moves from the reference

marks.

2) After detailed contouring, the final isocenter position is

marked on the patient.

The Ct Sim For Patient Marking

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XIO FocalSim


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3D CT Images

• Scanned =

Transverse/

Axial

• Generated=

Sagittal

• Generated=

Coronal

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Virtual Simulations

• Ct Sims allowed the verification

simulation process to become

virtual

• Traditional 2D fields could be set in

the 3D dataset

• The patient could be scanned,

marked and sent home given an

appointment time for treatment

XIO Focalsim

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Advantages of the CT Simulator

• Virtual simulation/verification process

• Generating a new/conedown plan can

be accomplished without having to

bring the patient back for another

simulation.

Disadvantages of the CT Simulator

Structure motion cannot easily be

detected with a CT Simulator. Excluding

4D scanners.

• CT doughnut is usually restricted to 70-

80cm in diameter .This can limit the

patient's position for some treatments. For

example, placing the patient's arms up can

be a problem.

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CT Simulators

CT scan process allows 3D volumetric

information to be gathered and carries

out simulation as a digital process.

Relies on construction of digitally

reconstructed radiographs

QA of CT-Simulators

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HISTORICALLY: QA of CT Scanners

• CT scans for treatment planning are often done with a flat top insert on the CT table to reproduce the radiation therapy treatment couch top.

• laser system mimicking that used on the simulation and treatment units

should be mounted in the CT suite and the alignment of the lasers should be checked daily. Such a system is an integral component for relating the patient’s position during CT with that on the simulation and treatment machines.

• The correlation of CT numbers with electron densities and the variation of CT numbers with position and phantom size should be determined. Since this correlation is a function of the quality of the x-ray beam, it should be

checked yearly.

• In addition, the CT scanner should be checked for image quality and other parameters described in the QA protocol provided by the manufacturer.

• QA of CT scanners (AAPM, 1977).

2003

• Quality assurance for computed-tomography

simulators and the computed tomography-

simulation process:

• Report of the AAPM Radiation Therapy

• Committee Task Group No. 66

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AAPM Task Group #66

Mechanicals

Common Sense Applies

• +/- 2mm most items

• Table indexing and motions

are 1mm

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Spatial integrity

QA goals: CT-simulation images should

accurately reproduce true patient anatomy

within 1 mm without spatial distortions in the

entire scan field. This should be verified for both

head and body scan protocols using a phantom

of known dimensions.

Spatial resolution

• Characterizes the imaging system’s

ability to distinguish between two very small

objects placed closely together.

• Spatial resolution is frequently referred to as

high contrast resolution

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High contrast resolution

• most commonly measured using either a resolution pattern ~line pair phantom with a range of spatial frequencies!, or by the modulation transfer function ~MTF! method.

• The line pair pattern in following slide ranges in frequency from 1 lp/cm to 21 lp/cm. Note the Bead in the phantom phantom which is a high-density, tungsten carbide bead which is used to create an impulse, or point source, from which the MTF can be calculated.

• Manufacturers often specify the limiting spatial resolution at the 5% or lower point on the MTF curve. The limiting spatial resolution ~lp/cm measured with MTF, and specified at the 5%value, is typically higher than the resolution that can be observed with a line pair phantom.

CT Scanner Line Pairs

• Slide Images from http://health.siemens.com/ct_applications/somatomsessions/index.php/image-quality-in-computed-tomography-2/

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MTF

• Plots the contrast

against the

resolution

• Completely

characterizes the

high-contrast

resolution of the

scan mode


High

Contrast

Low

Contrast

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Contrast resolution

• Contrast resolution can be defined as the CT-scanner’s ability to distinguish relatively large objects which differ

only slightly in density from background.

• QA goals: Quality assurance should demonstrate that the CT-scanner meets or exceeds manufacturer specifications for

low contrast resolution

Sensitivity and Profiles


Slice sensitivity is a curve

showing the effect of broadening

of the CT slice

Thickness along patient in helical

CT

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Image Performance

When referencing

manufacturers

specifications

tolerances are set to

the acceptance criteria

and

can then be called the

baseline measurement

CT Sim Software:

• Image input test

• Structure delineation (contouring)

• Multimodality image registration

• Machine definition

• Isocenter calculation and movement

• Image reconstruction

• Evaluation of digitally reconstructed

radiographs

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Evaluation of digitally

reconstructed radiographs

• Spatial and contrast resolution: It is generally understood that

smaller slice thickness and spacing produces better spatial resolution DRRs.

• Geometric and spatial accuracy: Magnification should be within 1

mm of expected. Spatial errors ~e.g., collimator, table rotation, incorrect jaw

setting, etc.! can also cause errors which may not be detected from patient

port films. The QA for the CTsimulation process should include evaluation of

DRR geometric errors.

• Hardcopy quality: Printing of standard test patterns and

comparison with baseline data can reveal potential problems

EVALUATION OF THE

CT-SIMULATION PROCESS

• Overall process tests: Patient positioning and

immobilization, Scan limits, Scan protocol,

Contrast, Special considerations and

instructions, Data acquisition,

Localization/marking, Virtual simulation, DRR

and setup documentation

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DRR

• Digitally constructed images from the

3D dataset

• Generated with the same geometry as

a divergent radiograph produced with a

point source of radiation.

• Created by combining the influence

of the CT pixel elements from a CT

dataset along divergent ray lines.

DRR Tools

• Computer generated films allow

selection of the bony anatomy

needing to be imaged

• DRR’s can be adjusted by using a

window/leveling tool

• Drr’s can be generated for any

treatment angle there are no

collision issues in virtual space

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DRR Region of Interest

DRR Quality

• # of Slices

• CT # accuracy

• Slice thickness

• Scan technique used

• Reconstruction algorithm

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DRR Artifacts

• Contrast Agents

• Prosthesis

• Respiratory Motion

• Anatomy (inadequate scan technique)

Scanner Types

• First Generation: Translate/Rotate

• Second Generation:

Translate/Rotate

• Third Generation: Rotate/Rotate

• Fourth Generation: Rotate/Fixed

• Spiral CT (3rd or 4th generation

type)

• Cine Ct

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•Retrieved from http://www.impactscan.org/download/msctdose.pdf

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Single Slice Spiral CT Pitch

Pitch = (table increment

distance (mm) per 360° gantry

rotation) / slice thickness (mm)

slice thickness 5mm, table

motion 7.5mm/rotation , Pitch

= 1.5

Pitch of 1=adjacent rotations

Pitch>1 = gaps between x-ray

beams from adjacent rotations

Multi Slice Spiral CT Beam Pitch

= (table increment distance (mm) per

360° gantry rotation) / slice

thickness (mm) X n (number of

slices acquired)

slice thickness 5mm, on a 4 slice

scanner, table motion 15

mm/rotation

Beam pitch = 15 / 4 x 5 = .75

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Slice Sensitivity

Image Reconstruction

• Iterative Solvers-slower but better when

missing data

• Analytic Methods-Fourier Analysis

• Filtered back projection

•Can begin with first data acquired

•Can be “hard wired” into system

(speed)

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Image

Reconstruction

Ramp

Filters

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Cone Beam CT

• Planar images are acquired with the kV or MV

imaging system.

• Volumetric image reconstruction is performed

Houndsfield Units

The relative attenuation coefficient ()

is usually expressed in HU aka CT

numbers

HU= 1000 x (x- water)/ water where x

is the attenuation coefficient of

material x and water is the

attenuation coefficient of water

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HU vs CT

CT Numbers are based on manufacturer

constant “K”

CT = K x (x- water)/ water where x is the

attenuation coefficient of material x and water

is the attenuation coefficient of water

Houndsfield Units

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Window

and

Level

Lung Windows

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Soft Tissue Windows

Relation of FOV, Matrix

Size and Pixels

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Beam Hardening

Figure 15b. CT images of a patient with metal spine implants, reconstructed without any correction (a) and with metal artifact reduction (b).

Barrett J F , Keat N Radiographics 2004;24:1679-1691

©2004 by Radiological Society of North America

http://radiographics.rsna.org/content/24/6/1679/F23.large.jpg

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Partial Volume Effects

Motion

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Typical Doses

Magnetic Resonance Imaging

• Study of the magnetic properties

of the nucleus

• Nuclei under a strong magnetic

field absorb energy which is

then released at a later time

• This time period is unique to the

nuclei and surrounding area

• T1 and T2 are time values

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T1 Images

T2 Images

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MRI: The Pros and Cons

Pros: 1. Better soft tissue imaging

2. Multiplane imaging

3. Data unaffected by bones

Cons: 1. Image distortion

2. No electron density

information-cannot be used for

dose calculation w/o CT fusion

Positron Emission Tomography

• Functional images: provides information

about physiology instead of anatomy

• Generates transverse images depicting the

distribution of positron emitting nuclides

• MUST be fused with CT images for

treatment planning

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PET Continued

When positron annihilates it emits two

511keV photons in nearly opposite

directions; these photons interact with the

annihilation coincidence detectors and

obtain projections of the activity

distributed in the patient

Image Fusion – 4 Techniques

1. Coordinate transformation

Fiducial markers/stereotactic frames

2. Surfaced based registration

The surfaces of one or more structures are matched and used for computation and minimizing mismatch of the data set. Useful with skull or pelvis.

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Image Fusion Continued

3. Image Based Registration

Grayscale data is used directly to measure mismatch or similarity between datasets (Mutual Information-measurement of redundant data)

4. Interactive Techniques

Effective in cases with a limited number of degrees of freedom. Verified visually. Can be used to limit the amount of time needed for calculation based fusion.

Fusion

• Once the datasets are fused structures may

be mapped from one dataset to another

• So target volumes may be delineated on an

MRI or PET and transferred to CT data for

planning

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MRI/CT Mutual Information

MRI/CT Grayscale Visual Check

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PET/CT Pre-Fusion

PET/Ct Post Fusion

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What is the fourth dimension?

• Time and therefore motion

4D CT Scan Measures Lung Cancer Motion

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4D CT scan

•GE Lightspeed with

Varian RPM system

captures repeat CT

images at each couch

position during respiratory

cycle

•CT sample

interval 15-20

images/slice

position

Pan et al, Med Phys 31, 333 (2004); Med Phys 34, 4499 (2007)

4D thoracic CT imaging

Vedam et al PMB 2003

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What use are 4D CT scans?

• Determine tumor motion/screening

• Motion inclusive treatment

• Respiratory gated treatment

• 4D radiotherapy

All video images on 4d treatment techniques are curtesy of Paul Keall

4D CT in radiotherapy

• Scenario 1: No respiratory motion management

devices

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Exhale Inhale

Tumor

Inhale & exhale CT phases

Tumor

Motion

encompassing

volume

Motion inclusive treatment

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Scenario 2: Respiratory gating

• Acquire 4D CT

• Select respiratory phase(s)

• Delineate GTV/CTV on chosen phase(s)

• Create PTV

• Plan and treat with gating

Beam ON Beam ON Beam OFF

tumor

tumor

tumor

Gating

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Respiratory gated treatment

4D Radiotherapy

• Accuray

• Works in Progress

– Dynamic MLC motion to match target motion

– Dynamic table motion

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Things to Chew Over

• Dynamic delivery will require planning

of each phase of respiration

• What will the QA of the delivery devices

look like

• 2010 question- What if the patient

sneezes?!?

• 2011 answer- 4D conebeam

2011 - 4D CBCT – Elekta

XVI 4.5 Symmetry • Slow gantry motions – about 3 minutes for a 200 degree

rotation

• Software auto correlates data by surface or internal motions

• Motion induced blur of structures reduced

• Streaking artifacts common-more visible in axial images

• Have had patients not treated due variations in respirations – usually caused by coughing from illness-returned next day with cough suppressant

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•Sonke et al, Med Phys 32, 1176 (2005)

4D-CBCT Streaking

•Li & Xing, IJROPBP 67, 1211, 2007

Streaking artifacts can be

Reduced with slower gantry

Rotations = increased times

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4D CBCT Pre-Treatment

Verification

• Thank you so much for your time and

consideration

• Good luck on all your future physics endeavors

patient positioning - scott memorial...

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