in vivo patient dose verification of volumetric modulated arc therapy including stereotactic body

218 Pages·2016·2.56 MB·English

in vivo patient dose verification of volumetric modulated arc
therapy including stereotactic body radiation treatment
applications using portal dose images

by
Peter Michael McCowan

A thesis submitted to the Faculty of Graduate Studies of
The University of Manitoba
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY

Department of Physics & Astronomy
University of Manitoba
Winnipeg, Canada

Copyright © by 2015 Peter Michael McCowan
ACKNOWLEDGEMENTS

This work would not have been completed without financial support. I have received
funding for my research from the CancerCare Manitoba Foundation, the University of
Manitoba’s Faculty of Science, and the University of Manitoba’s Faculty of Graduate
Studies.

I would like to thank my advisory committee: Dr. John Lewis, Dr. Francis Lin, Dr.
Daniel Rickey, Dr. Lawrence Ryner, and Dr. Gabriel Thomas for their help and support over
past five years. More importantly, I would like to thank my supervisor Dr. Boyd McCurdy
for introducing me to the field of radiation physics as well as his enthusiasm, dedication,
insight, friendship, and leadership during the course of my PhD studies.

I would like to thank some CCMB staff: Ken Allen and Chris Dyke from nuclear
electronics, Bob Miller and Todd Boyer from the fabrication shop, staff physicists Dr. Eric
vanUytven, Dr. Ryan Rivest, and Dr. Dave Sasaki for their help in acquisition, analysis, and
trouble-shooting.

I would like to thank the past and present students whom I’ve had the chance to meet
during the course of my PhD: Dr. Jorge Alpuche, Dr. Ganiyu Asuni, Heather Champion,
Tamar Chighvinadze, Dr. Krista Chytyk-Praznik, Hongyan Sun, and Troy Teo. A special
thanks as well to Timothy Van Beek for his intelligent contributions and code writing
expertise.
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I would like to very much thank my parents, Rudi and Janet, and my brothers Alexander,
Angus, and James for their support and encouragement over the years. I would like to
especially thank my fiancée Jennifer Vancuren for her love, understanding, patience and
overall amazingness over these past years. Finally, a thank you to my good friends Rylan
Page, Devyn Rolke, Jason Senyk, and Chad Slobodian for keeping me level and sane
throughout.

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ABSTRACT

The complexity of radiation therapy delivery has increased over the years due to
advancements in computing and technical innovation. A system of dose delivery verification
has the potential to catch treatment errors and therefore improve patient safety. The goal of
this thesis was to create a portal image-based in vivo dose reconstruction model for
volumetric modulated arc therapy (VMAT) deliveries, specifically for stereotactic body
radiation therapy (SBRT). This model-based approach should be robust and feasible within a
clinical setting. VMAT involves the modulation of dose rate, gantry speed, and aperture
shaping while the treatment gantry (i.e., x-ray beam) rotates about the patient. In this work,
portal images were acquired using an amorphous silicon electronic portal imaging device (a-
Si EPID).

A geometrical characterization of the linear accelerator (linac) during VMAT delivery
was performed. The effect of gravitational sag on the EPID imager and EPID support arm
was experimentally determined to be a reproducible effect and thus correctable. The angular
accuracy of the EPID images acquired during a VMAT arc was investigated and found to be
erroneous by as much as 3°. An angle adjustment method was determined which improves
each EPID’s angular accuracy to within ±1° of the true physical angle. An accurate EPID
image angle improves the accuracy of our in vivo dose reconstruction.

An in vivo patient dose reconstruction model for conventional intensity modulated
radiation therapy and, more recently, VMAT has been developed. This thesis focuses on
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adapting the model for SBRT-VMAT treatments. SBRT delivers large doses over fewer
fractions than conventional radiotherapy, therefore, any error during an SBRT delivery will
have a greater impact on the patient. This strongly encourages an in vivo dosimetry
verification system. In this work, a robust, model-based SBRT-VMAT dose reconstruction
verification system using EPID images was developed. This was validated using seven
different patient plans delivered to an anthropomorphic phantom as well as two plans
delivered to patients. The model was determined to be clinically feasible.

Each EPID image saved by the clinical computer is the average of an integral number of
frames. The accuracy of a 3D in vivo dose reconstruction, using all the EPID images acquired
during treatment, is sensitive to the chosen frame averaging per EPID image: the greater the
frame averaging, the larger the reconstruction error. However, the more EPID images, the
greater the calculation time. Optimization of the EPID frame averaging number as a function
of average linac gantry speed and dose per fraction were determined.

The EPID-based in vivo dose reconstruction model for SBRT-VMAT developed here was
determined to be robust, accurate, and clinically feasible as long as adjustments were made in
order to correct for EPID image geometrical errors and frame-averaging errors.
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TABLE OF CONTENTS
Acknowledgements………………………………………………………………………… ii
Abstract…………………………………………………………………………………….. iv
List of Tables……………………………………………………………………………….. ix
List of Figures…………………………………………………………………………….... x
List of Copyright Material…………………………………………………………………. xiii
Contributions to Science…………………………………………………………………… xiv
CHAPTER ONE: RATIONALE
1.1 An Overview of Radiation Therapy……………………………………………. 1
1.2 Dosimetric Accuracy……………………………………………………………4
1.3 Accidents and Errors in Radiation Therapy…………………………………….6
1.4 in vivo Dose Verification for Radiation Therapy Treatments…………………..8
1.5 Hypothesis………………………………………………………………………8
CHAPTER TWO: INTRODUCTION
2.1 Linear Accelerator Operation…………………………………………………...11
2.2 Modeling Linac Fluence……………………………………………..………….18
2.3 VMAT Dosimetry Devices………………..……………………………………. 33
2.4 Portal Imaging………………………………………………………………….. 41
2.5 Portal Dosimetry……………………………………………………………….. 46
2.6 Thesis Summary………………………………………………………………... 54

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CHAPTER THREE: THE CHARACTERIZATION OF GRAVITATIONAL SAG ON AN EPID
DURING VMAT DELIVERY
3.1 Introduction…………………………………………………………….. ………65
3.2 Materials and Methods……………………………………………………….....67
3.3 Results and Discussion………………………………………………………….72
3.4 Conclusions…………………………………………………………………….. 76
CHAPTER FOUR: AN INVESTIGATION OF GANTRY ANGLE DATA ACCURACY FOR
CINE-MODE EPID IMAGES ACQUIRED DURING VMAT
4.1 Introduction……………………………………………………………………. 77
4.2 Materials and Methods………………………………………………………… 81
4.3 Results and Discussion………………………………………………………… 95
4.4 Conclusions…………………………………………………………………….. 107
CHAPTER FIVE: AN in vivo DOSE VERIFICATION METHOD FOR SBRT-VMAT
DELIVERY USING THE EPID
5.1 Introduction……………………………………………………………………. 110
5.2 Materials and Methods…………………………………………………………. 112
5.3 Results and Discussion………………………………………………………… 120
5.4 Conclusions…………………………………………………………………….. 129

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CHAPTER SIX: FRAME-AVERAGING OPTIMIZATION OF CINE-MODE EPID IMAGES
USED FOR in vivo PATIENT VERIFICATION OF VMAT DELIVERIES
6.1 Introduction……………………………………………………………………. 133
6.2 Materials and Methods…………………………………………………………. 135
6.3 Results and Discussion………………………………………………………….143
6.4 Conclusions…………………………………………………………………….. 152
CHAPTER SEVEN: SUMMARY
7.1 Summary……………………………………………………………………….. 155
7.2 Conclusions…………………………………………………………………….. 160
7.3 Future Work……………………………………………………………………. 161
CHAPTER EIGHT: APPENDIX
8.1 The Physics of Therapeutic X-rays…………………………………………….. 166
8.2 Dose Image Comparison Techniques…………………………………………...187
8.3 Validation of the CCMB Collapsed Cone Convolution Algorithm……………. 191
8.4 Glossary………………………………………………………………………....197
8.5 Permissions for Copyrights…………………………………………………….. 202

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LIST OF TABLES

CHAPTER FOUR
4.1 Encoder and Gantry-Angle Phantom Absolute Mean Differences in Angle
With the Linac Potentiometer……………………………………………………… 101
4.2 Differences between the Potentiometer and EPID Header Angles,
Including Time Delays and Linear Fits…………………………………………….. 103

CHAPTER FIVE
5.1 SBRT Treatment Delivery Summary……………………………………….......120
5.2 Summary of Dose Calculation Comparisons…………………………………... 126

CHAPTER SIX
6.1 VMAT Treatment Delivery Details……………………………………………. 137
6.2 Frame Averaging Optimization Results ………………………………………..148

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LIST OF FIGURES

CHAPTER ONE
1.1 TCP vs NTCP………………………………………………………………….. 6
CHAPTER TWO
2.1 Linear Accelerator………………………………………………….…………...14
2.2 Bremsstrahlung Spectrum……………………………………………………… 15
2.3 Flattening Filter Effect…………………………………………………………. 16
2.4 Water Tank Profiles……………………………………………………………..17
2.5 Fluence Functions……………………………………………………………… 25
CHAPTER THREE
3.1 Linac Experimental Setup……………………………………………………… 68
3.2 EPID Sag Geometry…………………………………………………………… 69
3.3 Isocentre Misalignment Corrections…………………………………………… 72
3.4 EPID y-axis Sag………………………………………………………..………. 74
3.5 EPID x-axis Sag………………………………………………………………... 74
CHAPTER FOUR
4.1 Signal Acquisition……………………………………………………………… 82
4.2 Experimental Setup at the Linac……………………………………………….. 83
4.3 Gantry-Angle Phantom………………………………………………………… 84
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