PRACTICAL Radiation Oncology Physics PRACTICAL Radiation Oncology Physics A Companion to & GUNDERSON TEPPER’S CLINICAL RADIATION ONCOLOGY Sonja Dieterich, PhD, DABR Associate Professor Department of Radiation Oncology University of California Davis, California Eric Ford, PhD, DABR Associate Professor Department of Radiation Oncology University of Washington School of Medicine Seattle, Washington Dan Pavord, MS, DABR Chief Medical Physicist Radiation Oncology Health Quest Poughkeepsie, New York Jing Zeng, MD, DABR Assistant Professor Department of Radiation Oncology University of Washington School of Medicine Seattle, Washington 1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899 PRACTICAL RADIATION ONCOLOGY PHYSICS: ISBN: 978-0-323-26209-5 A COMPANION TO GUNDERSON & TEPPER’S CLINICAL RADIATION ONCOLOGY Copyright © 2016 by Elsevier, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data Dieterich, Sonja, author. Practical radiation oncology physics: a companion to Gunderson & Tepper’s Clinical radiation oncology / Sonja Dieterich, Eric Ford, Daniel Pavord, Jing Zeng. p. ; cm. Per publisher’s web site, this book is a companion to the fourth edition of Clinical radiation oncology, by Drs. Leonard Gunderson and Joel Tepper. ISBN 978-0-323-26209-5 (pbk. : alk. paper) I. Ford, Eric (Radiation oncologist), author. II. Pavord, Daniel, author. III. Zeng, Jing (Radiation oncologist), author. IV. Clinical radiation oncology (Gunderson). 4th ed. Supplement to (expression): V. Title. [DNLM: 1. Radiation Oncology. 2. Physics. QZ 269] RC270.3.R33 616.99′40757—dc23 2015013608 Senior Content Strategist: Suzanne Toppy Senior Content Development Specialist: Dee Simpson Publishing Services Manager: Hemamalini Rajendrababu Senior Project Manager: Beula Christopher Design Manager: Amy Buxton Illustrations Manager: Karen Giacomucci Marketing Manager: Deborah Davis Printed in China Last digit is the print number: 9 8 7 6 5 4 3 2 1 Dedications For Sigrid “Witzi” Dieterich, 1950–1996. —Sonja Dieterich To my wife, Elisa. Your support and love is everything. —Eric Ford To my family—Anna, Craig, Lillian, and Scott. To my friends and mentors—Mitch, Kris, and Peggy. —Dan Pavord For my husband, parents, and mother-in-law who made this possible. —Jing Zeng F O R E W O R D There are few fields in medicine for which the role of the radiation physicist is as important as it is in radiation oncology. Our machines are designed by engineers based on sophisticated physi- cal principles, machines are commissioned in individual institutions by physicists and engineers to ensure that they meet exacting tolerance, and daily treatments are designed by dosimetrists and implemented by the entire treatment team. As new clinical approaches are developed they need to be implemented in individual clinics, with modifications necessary for the idiosyncrasies of the equipment and needs of each department. For this to be done well, the physics team must have a strong knowledge of the basic principles involved as well as the design of the hardware and software packages. As radiation therapy has become far more complicated, the expectations that the physicians have of the physics team have escalated accordingly. The days are long past when central axis dose calculations were sufficient, and when imaging was never more than a weekly port film and computer software was rudimentary. Thus, quality control has become critical in all areas of radiation therapy delivery because there is so much more that can go wrong. Systems need to be put in place in each institution to ensure, prior to treatment delivery, that the treatment that the physician intended is indeed what was delivered to the patient. The critical nature of this work is made even harder by the very small margin of error that is routine in radiation therapy treatment delivery. The physician is usually prescribing radiation doses that are at the edge of what the normal tissues will tolerate, and at the edge of what is needed for best tumor control. Any substantial deviation from those doses is likely to lead either to complications or loss of tumor control. Thus, a book such as this that covers these areas well is important to the radiation oncology community. This new handbook is intended to provide a concise and practical summary of current standards of practice in radiation therapy physics and is appropriate for medical physicists, radia- tion oncologists, and residents. This book covers the topics needed for high-quality physics support at any contemporary institution in a practical and understandable manner. It provides far more than texts that limit themselves to simply describing the physical aspects of radiation therapy delivery, but also has a strong emphasis on the quality and safety issues that have come to the forefront in the past number of years. The fact that this book is written by four individuals allows for consistency in terms of writing style and approach and helps avoid duplication or missing topics of importance. It will be a valu- able resource to allow patient care to proceed at the highest level. Joel E. Tepper, MD, FASTRO Hector MacLean Distinguished Professor of Cancer Research Department of Radiation Oncology University of North Carolina Lineberger Comprehensive Cancer Center University of North Carolina School of Medicine Chapel Hill, North Carolina vi P R E F A C E Every so often, an email lands in our inboxes asking for help in getting started with a new clinical program; often these are questions about radiosurgery, brachytherapy, prone breast, use of CBCT, and many other new clinical procedures. As medical physicists responsible for implementing new technologies, we have many questions: Are the vendor recommendations sound? What professional society guidance documents are available, and are they still describing the current best practices? Is the method described in the recent paper by a major research university some- thing I can follow with the resources I have at my smaller clinic, or is it not quite ready for prime time yet? At present there are no standard resources available to answer these questions. We looked at our bookshelf of great didactic works and wished there was a book out there we could use to convey the basic overview of current best clinical practice and a curated reference list of guidance documents and publications to deepen the knowledge. The medical residents all carry such a book for radiation oncology in their coat pockets; why do we not have one for medical physics? Each of us approached the others about finding solutions to these questions. We quickly realized that we were focusing on the same topic, and thus the idea of this book was formed. And then came Hurricane Sandy. Trapped in Boston with the ASTRO meeting temporarily suspended, we pitched the book proposal, and in Elsevier found a publisher who also saw the need and was ready to support our idea. The goal of this book is to promote the safe and effective treatment of patients by providing an easily accessible distillation of all the current background and recommendations for best practices in the field. We have drawn from society recommendations, high-impact papers, and our own clinical experience absorbed from our mentors and learned in the trenches. Of the many ways to organize the material, we chose the following approach. The first twelve chapters cover the technical basics of the major clinical areas. Roughly, those are all the tasks physicists perform on phantoms, without a patient being in the room. The second half of the book covers major clinical treatment modalities. The work done in those areas directly involves patients and requires teamwork from the multiple professions that make up the patient care team. There are often several valid approaches to achieve the treatment goals, and the standard of care is changed and improved as new publications provide evidence for changing practice. Each book chapter provides a short introduction, followed by several sections providing an overview of the current best clinical practice(s). Recommendations from the various guidance documents published by IAEA, ACR, ASTRO, AAPM, ABS, and other professional organiza- tions or regulatory bodies are summarized and referenced as applicable. A “For the Physician” section concludes each chapter. This section summarizes the most important points with the goal to enable physicians to quickly check whether the physics work performed under their supervision is in agreement with safety and quality standards. How should you use this book? Physics residents and junior physicists just entering the clinic will find the chapters to be introductions to their clinical rotations. Medical residents might want to focus on the clinical chapters to gain insight on the technical and process components of patient treatments. Physicists setting out to implement a new procedure can focus on the appli- cable chapters of both the technical and the clinical sections of the book. Departments preparing for accreditation and physicians/physicists working on practice quality improvement projects will find the references in each chapter to be good starting points to evaluate their own practice against best practices recommended by their peers. And if none of the above apply, we the authors hope that in reading this book you will still find a few new pieces of information to expand your knowledge. As authors, we found the experience of learning and discovery while writing this book so rewarding that it well made up for the occasional difficult task of writing. vii viii Preface We hope that this book will prove to be a valuable investment of your time and money. Finally, we would love to hear from you. Do you know of a good guidance document we have missed? A clinical practice standard that should have been mentioned? What did we do well? After all, the best learning often does not come from books, but in the face-to-face discussion of ideas and concepts. Sonja Dieterich, PhD, DABR ([email protected]) Eric Ford, PhD, DABR ([email protected]) Dan Pavord, MS, DABR ([email protected]) Jing Zeng, MD, DABR ([email protected]) 1 C H A P T E R Reference Dosimetry for Ionizing Radiation 1.1 Introduction   The key to the accurate delivery of radiation is the ability to establish the absolute dose delivered. In radiation therapy clinical practice the primary tool used to measure absorbed dose is the ion chamber. The use of ion chambers has been well described by international codes of practice. While some of the details may differ slightly, the basic concepts of the various codes of practice are the same. The ionization measured by the chamber (typically filled with air) is converted to absorbed dose by applying a calibration factor (determined by an accredited calibration lab) and other correction factors based on the chamber design. The calibration factor may be a direct calibration in water (megavoltage (MV) photons, MV electrons, protons) or a calibration based on air kerma (kilovoltage (kV) photons, brachytherapy sources). The end goal is to determine absorbed dose to water in either case. The starting point for these calibrations is the absorbed dose standard developed at a primary standard dosimetry laboratory (PSDL). End users obtain a calibration factor for their equipment at a secondary standard dosimetry laboratory (SSDL), also known as an accredited dosimetry calibration lab (ADCL). The SSDL applies the standard developed by the PSDL using the available radiation sources at that lab. All SSDLs have 60Co sources available for calibration but may not have linac-generated beams. Some labs do have other high energy photon beams avail- able and can provide a calibration at multiple beam qualities. In the absence of this, correction factors must be applied to the calibration determined at 60Co energy to determine the calibration for the beam quality of interest. Further corrections are needed if the beam of interest is protons or other heavy ions. For a more complete discussion of the interaction between PSDLs, SSDLs, and the end user, the reader is referred to section 2.1 of International Atomic Energy Agency (IAEA) TRS-398, Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water.1 The regulations on the frequency of a full calibration in water may vary from country to country but are generally required at least once per year. The suggested regulations by the Conference of Radiation Control Program Directors (CRCPD) and implemented by many states in the United States require full calibrations at intervals not to exceed 12 months (section X.7.iii). More frequent constancy checks are required but can be done using solid phantoms and dosimetry equipment that is not calibrated by a calibration laboratory. These constancy devices should be compared with the calibrated system immediately after the annual full calibration. The equipment used for the full calibration should be sent for recalibration every 2 years. A constancy check should be performed before sending equipment to the calibration lab and after receiving it back to ensure that nothing happened during the process to change the response of the chamber. For new radiation therapy treatment machines, a second check of the absolute dose calibration should be obtained prior to treating patients.2 This could be accomplished by using 2 CHAPTER 1 Reference Dosimetry for Ionizing Radiation 3 a mail-order reference dosimetry service or a second check by a colleague using an independent dosimetry system. 1.2 Standard Megavoltage Photon Beams   Several international codes of practice are used to determine absorbed dose for MV photon beams, the American Association of Physicists in Medicine (AAPM) TG-51, Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams,3 IAEA TRS-398, Absorbed Dose Determination in External Beam Radiotherapy,1 Deutsche Industrie-Norm (DIN) 6800-2, Dosimetry Method for Photon and Electron Radiation—Part 2 Dosimetry of High Energy Photon and Electron Radiation with Ionization Chambers,4 Institute of Physics and Engineering in Medicine (IPEM) 1990, Code of Practice for High-Energy Photon Therapy Dosimetry,5 and others. An addendum to AAPM TG-51 has been published containing new k Q values.6 They are all based on absorbed dose in water calibrations of cylindrical ion chambers. The charge reading obtained from the ion chamber is corrected for temperature, pressure, ion recombination, and polarity. Corrections are then made to account for the perturbation to the medium (water) caused by the presence of the ion chamber. The calibration factor is then used to convert charge to absorbed dose. All of the protocols use a reference field size of 10 cm × 10 cm but the depth and source-to-surface distance (SSD) can vary among them. It is important to note that the reference depth for the calibration protocol is likely not the depth of absorbed dose specification in the clinic. For example, the protocol may specify measurement at 10 cm depth but the output of the machine is adjusted to 1.0 cGy/MU at depth of dose maximum (d ). This will require the use of accurate percent depth dose to correct the readings taken at max 10 cm depth to d depth. The general equation to calculate absorbed dose from a charge reading max of an ion chamber measured with an electrometer is: D (z)=M N k k k k (z) w D,w p s ρ Q where D (z) = absorbed dose to water at depth z w M = the ion chamber reading corrected for the electrometer calibration N = calibration factor for absorbed dose to water for 60Co energy D,w k = polarity correction. The value is generally less than 1% from unity. The value should be p stable from year to year and any deviation greater than 0.5% from the running average should be investigated. For new chambers, it should be measured several times to establish consistency. k = ion recombination factor. The value is generally less than 1.01 and AAPM TG-51 rec- s ommends not using a chamber if the correction is greater than 5%. The same recommen- dation regarding year-to-year stability applies. k = air density correction factor (often formulated as a temperature/pressure correction) ρ k (z) = beam quality correction factor for the measured beam versus the 60Co beam (in which Q N is determined). k is specific to the beam energy being measured and the ion chamber w Q used to make the measurement. Typical values range from 1 to approximately 0.96 for MV photon beams. A comparison of the four codes of practice is shown in Tables 1.1 to 1.5. Comparisons among the codes of practice show variations within the expected uncertainty levels of 1% to 1.5%.8-10 Standards labs are moving toward offering calibrations at beam qualities other than 60Co. This has the advantage that the k factor determined will be for the specific chamber used in the Q clinic, not a generic k for a given chamber model. There are two possibilities for implementing Q this strategy: 4 PART I Building Blocks TABLE 1.1 ■ Polarity Correction (k) Determination for the Various Protocols   p AAPM TG-51 (M+ −M− ) raw raw 2M raw IAEA TRS-398 (M+ −M− ) raw raw 2M raw DIN 6800-2 (M +M )/M/[(M +M )/M] 1 2 1 1 2 1Co IPEM 1990 none When determining polarity corrections and comparing among systems, care should be taken to understand the bias configuration used. This can vary among manufacturers. The value can either be less than or greater than 1 depending on which polarity is used for the final output determination but should not exceed 1%. In general k is a smaller effect with increasing energy. It should be noted that if k is known for the calibration beam quality p p (Q0), kp will then be determined as kp = kp,Q/kpQ0. It follows from this that if the user beam quality (Q) is the same as the calibration beam quality (Q0), kp = 1. This must be done if kp exceeds 0.3% for 6 MV or 60Co energies. TABLE 1.2 ■ Ion Recombination (k) Determination for the Various Protocols for Pulsed Beams   s AAPM TG-51 1−VH/VL MrHaw/MrLaw−VH/VL IAEA TRS-398 k =M1/M2−1 s U1/U2−1 or ks=a0+a1MM1+a2MM12 2 2 DIN 6800-2 U1/U2−1 U/U −[(M−M )k k ]/[(M−M )k k ] 1 2 0 p ρ1 0 p ρ2 IPEM 1990 1+X% for an X% decrease in reading when chamber voltage is halved Since MrHaw>MrLaw, the value is always >1. It should be noted that ion recombination is mostly dependent on dose per pulse and therefore will be different for flattening filter free beams.7 TABLE 1.3 ■  kρ Determination for the Various Protocols AAPM TG-51 273.2+T 101.33 × where T = water temperature in °C and P = pressure 273.2+22.0 P in kPa. Humidity must be between 20% and 80%. IAEA TRS-398 Standard temperature is 20 instead of 22. Therefore the denominator is DIN 6800-2 273.2 + 20.0 in the temperature term. IPEM 1990 1. The calibration lab determines N for the chamber along with a series of k values across D,w Q the range of beam qualities including electrons. This has the advantage that because the beam energy dependence for the chamber is not expected to change, future calibrations will only require determination of N at the reference quality, Q . D,w 0 2. The calibration lab determines a series of N values, eliminating the need for k . D,w Q