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CHAPTER 1

INTRODUCTION

1.1 Cancer and radiotherapy

Cancer is a term used for disease in which abnormal cells divides

without control and are able to invade other tissue. The main treatment

modalities include surgery, chemotherapy and radiotherapy. Radiotherapy

or radiation oncology is the medical use of ionizing radiation, generally as

part of cancer treatment to control or kill malignant cells. Ionizing radiation

works by damaging the deoxy ribo nucleic acid (DNA) of exposed tissue

leading to cellular death. Aim of radiotherapy is to give maximum

radiation dose to tumor while minimizing radiation to normal tissues to

reduce the complications. To spare normal tissues, shaped radiation beams

are aimed from several angles of exposure to intersect at the tumor,

providing a much larger absorbed dose there than in the surrounding

healthy tissue. The three main divisions of radiation therapy are external

beam radiation therapy or teletherapy, brachytherapy or sealed source

radiation therapy, and systemic radioisotope therapy or unsealed source

radiotherapy. The differences relate to the position of the radiation source:

external is outside the body, brachytherapy uses sealed radioactive sources

placed precisely in the area under treatment, and systemic radioisotopes are

given by infusion or oral ingestion.

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1.2 Radiotherapy machines and radiation treatment techniques

Therapeutic external radiation is given to the patients using the

equipments such as ortho-voltage units, deep-therapy x-ray machines, tele-

cobalt units and linear accelerators (linacs) which produce megavoltage x-

rays (figure1.1 and figure1.2). In treating with linear accelerators having

multileaf collimator (MLC), the treatment volume can be shaped to

conform to the tumor volume through beam shaping (figure1.3) and

shielding of normal tissues and critical organs.

Figure 1.1 A medical linear accelerator

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Figure 1.2 Medical linear accelerator and its components

Figure 1.3 Beam shaping with multi leaf collimators

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The technique of radiotherapy with beam shaping and shielding of

normal structures surrounding tumor volume is called as 3-dimensional

conformal radiation therapy (3-D CRT). Intensity modulated radiation

therapy (IMRT) is an advanced form of 3-D CRT (figure1.4). In IMRT,

customized radiation dose is intended to maximize tumor dose while

simultaneously protecting the surrounding normal tissue. The transition of

radiotherapy from IMRT to volumetric modulated arc therapy (VMAT)

made treatment of cancer easier & beneficial (figure1.5). In VMAT, three

parameters are changing simultaneously - Gantry speed of linac, MLCs

shape, and dose rate, but in IMRT there is no movement of gantry during

the treatment and there is no dose rate variation.

Figure 1.4 3DCRT and IMRT comparison

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Figure 1.5 IMRT and VMAT comparison

1.3 Importance of accuracy in dose planning and delivery in IMRT

It is well understood in radiation therapy that the dose-response

curves are quite steep and there is clinical evidence that a small change

(5%) in the dose to target volume can result in a change in the tumor

control probability (ICRU 1976). Along the same argument, similar dose

change may also result in a sharp change in the incidence and severity of

radiation-induced morbidity, especially for serial critical structures such as

spinal cord, optic chiasm, and brain stem. For IMRT and VMAT the

accuracy in dose planning and delivery is even more important because

even a small displacement of the delivered dose distribution can result

changes in doses that exceed the tolerance values for critical organs and

seriously under dose the tumor volume. Based on clinical evidence on

effective and excessive dose levels, the consensus in radiation therapy

community is that the dose delivered to the tumor volume should be within

5% of the prescribed dose. Therefore, the guiding principle in establishing

quality assurance (QA) test procedures and in defining tolerance limits for

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IMRT process is to minimize the overall uncertainty in delivered dose to

less than 5%.

1.4 Overall process of IMRT, sources of errors and importance of

patient specific QA

The IMRT process comprises of several steps: treatment setup,

patient immobilization, computed tomography (CT) image acquisition,

inverse treatment planning, plan acceptance, plan verification, and the

actual treatment delivery. With multiple steps involved, there remains a

large potential for random and systematic errors at each step along the way.

Of these, the systematic errors are the most significant since they can have

a huge impact on the final treatment outcome. In a clinical scenario

therefore, every attempt should be made to reduce such systematic errors.

The first logical step is to analyze the uncertainties in both the components

of IMRT QA process – pertaining to machine (machine specific QA) and

pertaining to individual patient treatment (patient specific QA). In IMRT,

patient specific QA plays a crucial role because of the complexities

involved in treatment plan, dose calculation and treatment delivery.

1.5 Test tools and methods in IMRT patient specific QA

Commonly followed IMRT QA methods include point dose

measurements using a small volume ion chamber, planar dose

measurements using a film or a 2-D array detector, portal dosimetry etc

[(wagter et al. (2004)]. In advanced QA systems, fluence measured by a 2-

D array detector can be used to calculate dose to a 3-D volume. This makes

possible comparison between treatment planning systems (TPS) calculated

dose volume histogram (DVH) and QA system’s DVH.

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1.5.1 Point dose measurement

Absorbed dose determination using calibrated ionization chambers

in combination with a well established dosimetry protocol, such as the

international atomic energy agency (IAEA) protocol, are generally

assumed to be the gold standard in radiation dosimetry. Under reference

conditions, the estimated combined standard uncertainty in the

determination of absorbed dose in high energy photon beams amounts to

about 1.5%.

As an initial step, during the commissioning of IMRT, the solid

water phantom with ion chamber (figure1.6) has to be scanned and the

image set has to be imported to the treatment planning system. Patient

verification plans can be created in the treatment planning system for the

absolute dosimetric measurement by exporting the patient specific IMRT

plans on the image set of IMRT phantom, which saved in the treatment

planning system. After the 3-D dose calculation, the dose at a reference

depth in the phantom can be measured from the TPS created verification

plan. These plans will be executed in the linear accelerator. To measure the

absolute dose at the reference point, the IMRT water equivalent phantom

should place on the treatment couch and the ion chamber has to be inserted

at the level of reference point depth. The measured dose for each IMRT

fields at reference point is then compared with the TPS calculated absolute

dose at the same point and the % of variation will be calculated [Mijnheer et

al. (2008)].

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Figure 1.6 Solid water phantom, ion chamber and electro

meter for absolute point dose measurements

1.5.2 Film dosimetry

Radiographic films have been employed almost since the discovery

of X-rays to measure radiation dose. The use of radiographic films are

relatively easy, quick and cheap and therefore very often applied for many

applications in radiotherapy. It provides data with a high resolution and a

permanent record of the 2-D dose distribution in the plane of irradiation.

There are, however, many parameters influencing the film irradiation, film

processing and data analysis procedure that determine the accuracy of the

final result. Simultaneous dosimetric measurements in more than one point

have become an important need for quality assurance in modern

radiotherapy. Such measurements are traditionally performed with

radiographic films as a two-dimensional detector. However, their

application is not straightforward due to many factors of influence on the

optical density, such as energy and spectral composition, depth, field size,

orientation, and processing conditions [Kapulsky (2002)]. Additionally, the

increasing number of IMRT patients suggests the use of faster and more

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efficient dosimetric tools. Finally, many hospitals are aiming towards a so-

called “digital hospital”, where film-processing machines for traditional

silver-halide films will not be available or easily accessible in the near

future [Wiezorek et al. (2005)].

Radiochromic films, which are self-developing, almost tissue

equivalent and therefore shows little energy and directional dependence,

represent an alternative to radiographic films but their use is still limited

because they were until recently rather expensive [(wagter et al. (2004)].

Other factors include cumbersome film handling, sensitivity variations

across the film, and the low sensitivity to ionizing radiation doses typically

used in radiation oncology. This prevented their use in external beam

therapy, and their dominant application in radiation oncology was limited

to brachytherapy. Figure1.7 shows the film dosimetry set up for IMRT.

Figure 1.7 Film dosimetry set up for IMRT

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1.5. 3 Portal dosimetry

On-line electronic portal imaging devices (EPIDs) have been

developed for acquiring megavoltage images during patient treatment.

Megavoltage images, obtained in digital format with such a device, are

then used for further analysis, mainly for determining set-up errors. The

image information can, however, also be related to the dose delivered to

the EPID, yielding dose information in a plane instead of in one or few

points. EPIDs can serve for several purposes during the verification

process of IMRT and are used: 1) to verify the leaf position either during

static (step-and-shoot) or dynamic MLC (sliding window) techniques; 2)

to check the correct transfer of the leaf sequencing file to the treatment

machine; and 3) to measure the combined mechanical and dosimetric

performance of the treatment unit [Prisciandaroo et al. (2004), Fielding et

al. (2004), Yang et al. (2004)]. More recently the uses of flat panel

imagers based on amorphous silicon (aSi) are becoming more popular for

their use as 2-D dosimeters. Most new accelerators are nowadays

equipped with aSi1000 EPID and it can therefore be expected that the use

of these devices for IMRT verification will increase in the future. Several

approaches have been described for the use of EPIDs for pre-treatment

verification of IMRT delivery [Warkentin et al. (2003), Vieira et al.

(2004), Budgel et al. (2005), Monti et al. (2006), Winkler et al. (2006)].

More recently various groups have developed methods to translate EPID

images into 2-D primary fluence maps, which are then used as input in a

TPS to recalculate 3-D dose distributions using CT data of a phantom or

patient. Generally these approaches are able to reconstruct the 3-D dose

distribution in phantoms with a high accuracy.

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Figure 1.8 Linac with inbuilt aSi1000 EPID (black arrow)

The aSi1000 portal imager (figure1.8) is the most recent detector

used for portal dosimetry. It is a flat panel X-ray imager with large area

active matrix readout structure and is made up of phosphor or photo

conductor. This detector is having four major parts- 1mm Cu build up

plate, a scintillating Phosphor screen, Image forming sensitive layer and

associated electronics. The Cu build up plate absorb the incident photons

and emits recoil electrons and also it shields the scintillation screen from

the scattered radiation. The recoil electrons from the build up plate are

absorbed by the scintillating phosphor screen and convert it into visible

light. The image forming layer is a 512 X 384 matrix deposited on a glass

substrate. Here each pixel in the matrix is having 0.784 mm pitch and

consists of aSi-n-i-p photo cathode to integrate the incoming light in charge

capture and a thin film transistor (TFT). The associated electronics with the

TFT switches enables the charge capture readout. The image acquisition

system with fast readout electronics enable up to 30 frames per second is a

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major difference in aSi1000 EPID. The resolution of aSi1000 EPID is upto

0.39 mm.

Before using the EPID for clinical purpose, the dosimetric

calibration and characteristics study of the portal imager has to be

performed [Berger et al. (2006), McDermott et al (2006), Greer et al.

(2007)]. For the IMRT patient specific QA, verification plans are creating

in treatment planning system using PDIP (portal dose image prediction)

algorithm. To measure the delivered dose the aSi1000EPID has to be

placed at the calibrated distance from the source. The verification plan is

then executed in linac through the networking platform and control

console. The measured and TPS predicted planar doses for individual fields

can be compare using the portal dosimetry analysis tool (figure1.9).

Figure 1.9 Portal dosimetry work flow

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1.5.4 2-D array system

The major benefit of dosimeter arrays are their simple handling by

connecting them to a computer and the availability of on-line information.

Two-dimensional (2-D) arrays are more practical as they allow the

verification of a planar fluence or dose distribution. During the last years

several systems became commercially available for 2-D dosimetry. The

most commonly utilized dosimetric principles are ionization in air or

ionization in semiconductor material, but other principles such as

scintillation have been applied as well. Advantages of ionization chambers

are the simple calibration, practically no dead time, which allows real time

measurement, and no (significant) effect of radiation damage. In general,

dosimetric properties are governed by the physics principle of the detector.

The most important ones are dose linearity, energy dependence, directional

dependence, dose rate dependence, source to detector distance (SDD) field

size response and temperature response. The various commercially

available 2-D arrays show differences in the number of detectors, detector

spacing, detector shape, effective point of measurement, water-equivalent

build-up layer, backscatter layer, and maximum field size covered. Most of

these systems can be used for absolute dose measurement after appropriate

individual calibration procedures to correct for response variations across

the array. 2-D arrays mounted on the gantry enable IMRT verification at

gantry angles identical to the ones applied in treatment plans. For such

procedures detector misalignments and influences of gravity need to be

considered carefully and corrected for if present.

The commercial 2-D arrays for dosimetric purposes come with their

inherent evaluation software. It is generally possible to import calculated

dose distributions from a planning system and to perform 1-D dosimetric

analysis of profiles or a 2-D gamma evaluation using data of the whole

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array. The major limitation of 2-D array is it’s limited number of detectors,

which impairs measurements in high dose gradient regions and in small

fields. The usefulness of a gamma evaluation, based on dosimetric

information with a limited spatial resolution is therefore questionable.

Obviously the limited spatial resolution of the 2-D array influences the

effectiveness of the verification at some points. For that reason it is

recommended to combine the results of multiple measurements in which

the array has been replaced over a small distance. Recently transmission-

type radiation detectors have been developed that can be positioned on the

radiation entrance side of the patient. These detectors are multi wire or

multi-strip ionization chambers connected to a multi-channel electrometer.

They are designed to be placed in dedicated holders or in standard

accessory holders of the linear accelerator. As a consequence the spatial

resolution depends on the mounting distance. Because of their negligible

attenuation, transmission-type 2-D detectors can be permanently installed

on accelerators primarily used for IMRT. They enable on-line monitoring

of beam characteristics or leaf settings with and without the patient in

place. However, when using such detectors the characteristics of a certain

device need to be taken into account, including its specific influences on

the overall QA procedures and dosimetry logistics. Moreover, besides the

advantage of offering on-line information, 2-D detectors have the potential

to increase the overall efficiency for IMRT QA. In addition, these tools can

also be used for QA of linear accelerators used for conventional treatments,

such as measurement of leaf position, output constancy, beam symmetry

and field flatness.

There are different types of 2-D array systems available now in

market for the IMRT patient specific QA. ImatriXX 2-D array system of

IBA dosimetry, map check 2-D arrays of sun nuclear dosimetry system, 2-

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D arrays of PTW dosimetry system etc are the commonly using 2-D array

systems.

ImatriXX 2-D array verification-process

The ImatriXX 2-D array system consists of 1020 parallel plate ion chamber

arranged in a 32x32 grid, with an inter detector spacing of 7.619 mm. Each

detector is having a diameter of 4.5 mm, height 5 mm and chamber volume

0.02 cc. To compare the TPS calculated planar dose with the measured

planar dose the ImatriXX 2-D array system has to place on the treatment

couch of the linac with the detector level at 100cm from the source.

Sufficient backscatter is placing below the detector and build up is placing

above the ImatriXX detector (figure1.10).

Figure1.10 ImatriXX 2-D array system- measurement set up

The TPS created verification plans will be executed in the linac. The

measured and TPS calculated planar dose comparison can be performed by

the IMRT QA software (figure1.11). The gamma evaluation can be

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performed and the % of pixels passing the specified gamma criteria can be

obtained for every individual IMRT fields.

Figure1.11 ImatriXX 2-D array system – work flow

1.5. 6 Compass - DVH based verification system

Compass is a dose verification system of IBA and it is used in

combination with MatriXX-Evolution detector (1020 pixels) for pre-

treatment verification of conformal IMRT plans and with MatriXX -

Evolution and gantry angle sensor for the verification of rotational plans as

well as transmission detector (1600) pixels for online verifications of

IMRT plans and rotational plans. Compass can determine the 3-D dose

distribution in the patient anatomy, based on the measured beam intensity

and it determines the fluence for all segments in a beam. As this quantity

cannot be directly measured, Compass does first a calculation of the

expected response of electrical signal for each segment based on detector

pixel response, linac and detector models. After the measurement, expected

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and delivered responses are compared. The residual response is then used

for computation of the really delivered fluence. The dose computation in

Compass is a second independent step in which the resulting dose to the

patient is determined based on a collapsed cone super position algorithm.

For the commissioning of the Compass 3-D verification system with

MatriXX-Evolution detector, the same data of TPS commissioning

(profiles, depth dose curves, output factors, absolute dose measurements)

are used. The primary quantity determined by Compass is the fluence for

each segment. Discrepancies in delivery can be visualized as difference in

the response patterns. The fluence determined in Compass is then used as

input for the dose computation with the collapsed cone algorithm. For the

conformal or IMRT plans, the Dicom RT plan, Dicom RT dose, Dicom RT

structure and Dicom CT information etc are to be transferred to the

Compass verification system. The Compass will compute using collapsed

cone algorithm and compare the TPS calculated and compass calculated

DVHs (dose volume histograms) and provide the differences.

1.6 Tolerance limits and Action levels for IMRT verification

Tests for IMRT verification can be separated into those for

verification of equipment for IMRT delivery, verification of IMRT

treatment planning, and verification of patient-specific IMRT techniques,

i.e., of the combined planning and delivery process of that particular

patient treatment based on relative as well as absolute dosimetry. Different

approaches exist for the comparison of sets of measured and calculated

dose distributions [Mijenheer et al (2008)]. Each of these approaches needs

well-defined criteria for acceptance of a plan and procedures if these

criteria are not met. Tolerance and action levels can be used. These

quantities can be defined in the following way: whenever a parameter is

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found in the range below the tolerance level, the equipment is suitable for

high quality radiation therapy. If, however, a parameter exceeds the action

level, it is essential that appropriate actions be taken as soon as possible.

Consequently, tolerance levels are appropriate limits for performance

specification and for acceptance testing procedures, while action levels

might be regarded as more relevant values for use in ongoing quality

control activities. If a parameter has a value between the tolerance limit

and the action level, the responsible physicist will generally decide to

continue with the treatment until a suitable moment for further

investigation occurs. If such an investigation is not possible, then high

quality treatments should no longer be performed with such equipment.

Tolerance and action levels should now be defined for the various tests to

compare measured with calculated dose distributions. The most often

applied dose evaluation techniques comprise a direct comparison of dose

differences (%dose difference), a comparison of distance to agreement

(DTA) between measured and calculated dose distributions, and a

combination of these two parameters: the gamma evaluation method.

In the gamma evaluation, doses in the TPS calculated plan and the

measured at the same pixel position are compared, and the difference as a

percentage of the plan value is the percentage difference. In high gradient

regions, one looks for the distance between a pixel in the plan and a pixel

in the measured distribution, that have the same dose. This is the distance

to agreement (DTA).

Differences of about 5% are generally significant for IMRT

verification [Mijenheer et al. (2008)]. Deviations larger than ± 5% should

therefore firstly result in a review of the complete dosimetric procedure

taking into account the various factors influencing the comparison result. If

no explanation for the observed discrepancy can be given, the

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measurement may be repeated. A possible recommendation might then be

that a tolerance limit of ± 3% and an action level ± 5% should be applied

for these types of point dose verifications. When the number of comparison

points is large, simple methods of reporting deviations between dose

measurements and calculations will collapse, and a method of compiling

these deviations into a single number is required as a pass-fail criteria.

Other methods have therefore been proposed, e.g., the use of the quantity

“confidence limit” by Venselaar et al. (2001). The confidence limit is

based on the average deviation between measurements and calculations for

a number of data points in a comparable situation, and the standard

deviation (SD) of the average of the differences. The confidence limit is

then defined as the sum of the average deviation and 1.5 SD. The factor 1.5

was based on experience and a useful choice in clinical practice. A

multiplicative factor of 1.96 instead of 1.5 has later been proposed by Palta

et al. (2003) for having 5% of the individual points exceeding the tolerance

level. For both the verification of individual beams, as well as for the

verification of patient-specific “hybrid” plans, Palta et al. (2003) proposed

the set of values of confidence limits and action levels for IMRT

treatments. An IMRT treatment plan should not be used clinically if the

measured dose difference is more than the value given as the action level,

which serves therefore as a pass-fail criterion. Application of the gamma

evaluation method for selecting action levels is still in a development stage.

Careful statistical analysis of patient specific verification data might

reveal systematic uncertainties valid for the whole patient group. The

statistical analysis of the results of a routine QA programme, possibly

applied to a set of patients treated according to a class solution, can be very

useful in defining appropriate tolerance/action levels taking into account

the special aspects of IMRT relevant for a specific clinic. Currently no

recommendations for 3-D dose evaluation are available and are therefore

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urgently needed. Biological considerations, combined with the clinical

experience from the 3-DCRT era, may be required to develop tolerance and

action levels for the evaluation of 3-D dose distributions for an individual

patient.

If a gamma evaluation exceeds a certain action level for a chosen

combination of dose-difference and DTA criteria, then possible reasons for

discrepancies such as variation in phantom positioning and linac

performance should first be investigated. If these experimental

uncertainties are within accepted values, then it might be useful to repeat

the experiment to confirm the observed discrepancies. If the same areas of

the gamma maps fail the tolerance criteria again, then these areas should be

compared with the corresponding regions in the patient dose distribution,

and the implications of such a failure should be discussed with the

responsible physicist and radiation oncologist. For each patient a decision

should then be made if a new plan has to be generated or if the differences

are clinically acceptable. It should be noted that such patient-specific

action levels depend on many decisive factors, including the position and

size of the area that failed to pass the evaluation criteria, the dose level in

the PTV (planning target volume) or OAR (organ at risk), and the

sensitivity of the plan for movement. Furthermore, gamma evaluation is

currently mainly restricted to the dose delivered to the PTV, whereas the

dose in an OAR is equally important. Extension of decision protocols

including OAR is therefore urgently needed. The situation becomes even

more complicated if dose distributions are evaluated in 3-D. It should be

noted that tolerance limits and action levels have proven to be very useful

in everyday quality control of accelerators, but some parameters are not

easily and quickly corrected or repaired and some may almost be

impossible or very expensive to restore. On very rare occasions, it might

therefore be justified to use the radiation equipment clinically, even if an

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action level has been exceeded. The decision to clinically use a treatment

unit, in spite of the fact that an action level has been exceeded, has to be

discussed thoroughly and documented for every treatment method.

1.7 Motivation for research and purpose of the thesis

Quality assurance in IMRT is mainly founded on quantitative

comparisons between computed and/or measured dose distributions.

Differences between measurement and calculation are principally caused

by errors in treatment planning, patient positioning, treatment delivery and

radiation dose measurement technique. However a simple agreement

between the two distributions cannot be held as a proof of satisfying

quality. Indeed the distributions that are compared may contain individual

uncertainties or bias such that the agreement seen is a chance coincidence.

This consideration may serve as an argument to include many degrees of

freedom in the QA measurement process, i.e. in terms of measurement

points in the comparison, volume of detectors, resolution of detectors, type

or complexity of plans, number of fields or arcs, movement of carriage,

inclusion or exclusion of couch in the plan, mode of delivery etc. The

proposed QA tolerance values found in some reports for these parameters

should therefore be considered as general recommendations and these may

not be always achievable in a clinical scenario.

Such guidelines for the IMRT QA are given in the American

Association of Physicists in Medicine- Task Group - 119 (AAPM-TG 119)

report [(Ezzell et al. (2009)] and in the European Society of Radiotherapy

and Oncology (ESTRO) guidelines [(Mijenheer et al. 2008)]. But a

comprehensive report that considers all possible influencing parameters or

factors that affect QA results such as complexity of plans, degree of

modulation, numbers of target volumes, numbers of fields or arcs,

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movement of MLC carriage, inclusion or exclusion of treatment couch in

the plan, mode of delivery etc., is still to come. Because of the nature of

complexity and the many steps involved, an optimized stringent QA

protocol is essential to rule out all possible uncertainties and errors. This

research work mainly aimed to study the influence of different factors on

the optimization of patient specific QA in IMRT to adopt a stringent QA

protocol.

1.8 Aims and objectives

The main objectives of this research work were as follows:

- To assess the beam stability of a Varian high energy medical linear

accelerator; quantify MLC positional errors using ionometric

gravity test and dynalog file analysis.

- To establish the characteristics of the different QA systems such as

aSi1000 based portal dosimetry and ImatriXX 2-D array system.

- To study the angular response of the portal dosimetry and ImatriXX

2-D array system.

- To optimize the patient specific QA measurements for IMRT and

VMAT through the statistical analysis of the results.

- To examine the relationship and significance of different parameters

such as detector systems, types of plans, their complexity, number

of targets, movement of MLC carriage, number of fields or arcs,

inclusion or exclusion of couch insert etc., on the selection of pass-

fail criteria and action levels in patient specific QA.

- To examine whether the IMRT QA guidelines and published reports

can be used for the VMAT QA analysis also or separate

optimization is needed or not.

- To validate the Compass 3-D verification system and to optimize

patient specific QA for IMRT.

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1.9 Hypothesis

Null Hypothesis (H0)

The null hypothesis was that the patient specific QA results are not

influenced by the QA systems, type of plans, complexity of plans, number

of fields or arcs, number of target volumes, movement of carriage and

inclusion or exclusion of couch insert and there is no need for institutional

local optimization of QA for IMRT and VMAT.

Alternate Hypothesis (H1)

The alternate hypothesis was that the patient specific QA results are

influenced by the QA systems, type of plans, complexity of plans, number

of fields or arcs, number of target volumes, movement of carriage and

inclusion or exclusion of couch insert and institutional local optimization

of QA for IMRT and VMAT is needed.