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Tissue Inhomogeneity Correction for Megavoltage Photon Beams AAPM REPORT NO. 85 TISSUE INHOMOGENEITY CORRECTIONS FOR MEGAVOLTAGE PHOTON BEAMS Report of Task Group No. 65 of the Radiation Therapy Committee of the American Association of Physicists in Medicine Members Nikos Papanikolaou (Chair) University of Arkansas, Little Rock, Arkansas Jerry J. Battista London Regional Cancer Centre, London, Ontario, Canada Arthur L. Boyer Stanford University, Stanford, California Constantin Kappas University of Thessaly, Medical School, Larissa, Hellas Eric Klein Mallinckrodt Institute of Radiology, St. Louis, Missouri T. Rock Mackie University of Wisconsin, Madison, Wisconsin Michael Sharpe Princess Margaret Hospital, Toronto, Ontario, Canada Jake Van Dyk London Regional Cancer Centre, London, Ontario, Canada August 2004 Published for the American Asociation of Physicists in Medicine by Medical Physics Publishing DISCLAIMER: This publication is based on sources and information believed to be reliable, but the AAPM, the editors, and the publisher disclaim any war- ranty or liability based on or relating to the contents of this publication. The AAPM does not endorse any products, manufacturers, or suppliers. Nothing in this publication should be interpreted as implying such endorsement. Further copies of this report ($15 prepaid) may be obtained from: Medical Physics Publishing 4513 Vernon Blvd. Madison, WI 53705-4964 Telephone: 1-800-442-5778 or 608-262-4021 Fax: 608-265-2121 Email: [email protected] Web sit e: www.medicalphysics.org International Standard Book Number: 1-888340-47-9 International Standard Serial Number: 0271-7344 © 2004 by American Association of Physicists in Medicine One Physics Ellipse College Park, MD 20740-3846 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the publisher. Published by Medical Physics Publishing 4513 Vernon Boulevard, Madison, WI 53705-4964 Printed in the United States of America TABLE OF CONTENTS Abstract ..........................................................................................................v I. INTRODUCTION...............................................................................1 II. NEED FOR INHOMOGENEITY CORRECTIONS..........................3 A. Required Dose Accuracy ................................................................3 1. Slopes of dose-effect curves........................................................4 2. The level of dose differences that can be detected clinically .......4 3. The level of accuracy needed for clinical studies ........................5 4. The level of dose accuracy that will be practically achievable ...................................................................................8 B. Recommended Accuracy in Tissue Inhomogeneity Corrections .......9 III. RADIATION PHYSICS RELEVANT TO PHOTON INHOMOGENEITY CALCULATIONS ..........................................10 A. Photon Interactions: The TERMA Step.........................................10 B. Charged Particle Interactions: The DOSE Step .............................11 C. Charged Particle Equilibrium........................................................14 1. Longitudinal TCPE...................................................................15 2. Lateral TCPE ...........................................................................16 D. Influence of Tissue Density and Atomic Number ..........................16 1. Density scaling .........................................................................16 2. Effects of atomic number..........................................................22 E. Concept of Primary and Scattered Dose Components ...................24 F. Introduction to the Superposition and Convolution Principles........27 IV. INHOMOGENEITY CORRECTION METHODS ..........................29 Category 1: Local Energy Deposition (No Electron Transport); 1D Density Sampling..........................................................................32 Method 1.1: Linear attenuation .....................................................33 Method 1.2: Effective attenuation coefficient.................................34 Method 1.3: Ratio of tissue-air ratios (RTAR)...............................34 Method 1.4: Power law (Batho).....................................................35 Category 2: Local Energy Deposition (No Electron Transport); 3D Density Sampling..........................................................................38 Method 2.1: Equivalent tissue-air ratio (ETAR) ............................38 Method 2.2: Differential scatter-air ratio (dSAR)..........................44 Method 2.3: Delta volume (DVOL)...............................................44 Method 2.4: Differential tissue-air ratio method (dTAR)...............45 Method 2.5: 3-D beam subtraction method...................................46 Category 3: Non-Local Energy Deposition (Electron Transport); 1D Density Sampling..........................................................................47 Method 3.1: Convolution techniques .............................................47 Method 3.2: Fast Fourier Transform (FFT) convolution .................49 iii Category 4: Non-Local Energy Deposition (Electron Transport); 3D Density Sampling..........................................................................49 Method 4.1: Superposition-convolution methods ...........................50 Method 4.2a: Monte Carlo method: overview ...............................55 Method 4.2b: Monte Carlo: dosimetry in heterogeneous media.....58 V. DATA COMPARISON OF DOSE IN INHOMOGENEOUS MEDIA..............................................................................................65 A. Air Cavities...................................................................................67 B. Lung .............................................................................................69 C. Bone and High-Density Media......................................................75 D. Influence of CT Number Variations ..............................................77 E. Radiosurgical Beams.....................................................................78 F. Multiple Beam Arrangements........................................................79 G. Measured Benchmark Data ...........................................................80 H. Data Trends...................................................................................94 VI. THE EFFECT OF INHOMOGENEITY IN IMRT..........................95 VII. SUMMARY AND RECOMMENDATIONS ...................................100 VIII. REFERENCES................................................................................107 iv ABSTRACT The human body consists of a variety of tissues and cavities with different physical and radiological properties. Most important among these, from a radi- ation dosimetry perspective, are tissues and cavities that are radiologically dif- ferent from water, including lungs, oral cavities, teeth, nasal passages, sinuses, and bones. The dose distribution is affected by these tissue inhomogeneities and since treatments are becoming increasingly conformal, the opportunity for geo- graphic misses of the target due to incorrect isodose coverage increases. In view of the inconsistent use of inhomogeneity corrections, the recent advances in the dose calculation algorithms, the improved 3D image acquisition and display capabilities, and the trend towards dose escalation in smaller target volumes, the Radiation Therapy Committee (RTC) of the American Association of Physicists in Medicine (AAPM) commissioned Task Group 65 (TG 65) to review this subject specifically for megavoltage photon beams. The specific objectives of this Ta sk Group are: (a) to review the clinical need for inhomo- geneity corrections and the currently available methodologies for tissue inho- mogeneity corrections in photon beams; (b) to assess the known advantages and disadvantages of each of the currently available algorithms; (c) to make recommendations regarding the types of procedures that should be used to assess the accuracy of inhomogeneity correction procedures and the clinical application of specific inhomogeneity corrections for different anatomical regions. This report summarizes the findings of the Task Group and aims to provide the practicing clinical medical physicist with sufficient physical and mathematical insight into the inhomogeneity problem to be able to discern the capabilities and limitations of the particular method(s) available, to advise radi- ation oncologists as to the accuracy of the predicted doses and prescriptions, and to advise both so they are able to make informed decisions during the pur- chase of treatment planning systems. v I. INTRODUCTION The human body consists of a variety of tissues and cavities with different physical and radiological properties. Most important among these, from a radi- ation dosimetry perspective, are tissues and cavities that are radiologically dif- ferent from water, including lungs, oral cavities, teeth, nasal passages, sinuses and bones. In some instances, foreign materials, such as metallic prostheses, are also present. To maximize the therapeutic benefit of radiation therapy, it is essential that the absorbed dose delivered to all irradiated tissues in the pres- ence of such inhomogeneities be predicted accurately. Optimization of therapeutic benefit is dependent on maximizing the dose to the planning
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