NCRP REPORT No. 69 Dosimetry of X-Ray and Gamma-Ray Beams for Radiation iherapy in the Energy Range 10 keV - to 50 MeV Recommendations of the NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS Issued December 15, 1981 First Reprinting August 1, 1985 Second Reprinting May 15, 1989 Third Reprinting September 6, 1993 Fourth Reprinting August 15, 1995 National Council on Radiation Protection and Measurements 79l0 WOODMONT AVENUE / BETHLSDA, MD. #)814 LEGAL NOTICE This report was prepad by the National Council on Radiation Fkotection and Mea- aurements (NCRP). The Council atrives to provide accurate, complete and useful information in its reports. However, neither the NCRP, the members of NCRP, other penv~eco ntributing to or asakhg in the preparation of this report, nor any m n acting on the behalf of any of these partiea (a) makes any warranty or representation. expreas or implied, with respect to the accuracy, completeness or usefulness of the information contained in this report, or that the use of any informetion, method or process dieclosed in this report may not i&bge on privately owned rights; or (b) sswunes any liability with respect to the use of, or for damages resulting born the use of, any information, method or proceea disclased in this report. Copyri&t 8 National Council on Radiation Rotetion and Measurements 1981 All rights reserved. This publicntion is protected by copyright. No pnrt of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval eystem without written p d o n f rom the copmto wner. except for brief quotation in &tical artides or reviewa Library of Congress Catalog Card Number 81-81430 International Standard Book Number 0-913392553 Preface The purpose of radiation therapy is to irradiate a particular part of the human body with a prescribed absorbed dose with minimal expo- sure to other parts of the body. The absorbed dose to the treatment region should be accurate to some acceptable level in which the errom are substantially less than those variations in absorbed dose that produce unintended clinical effects. The objective of this report is to describe and recommend the dosimetric process that will allow the delivery of the prescribed absorbed dose from x-ray and gamma-ray sources to a uniform phantom to within this accuracy. Delivery of the correct dose to a patient (treatment planning), however, is not within the scope of this report. This report describes and discusses the m y recommended procedural details for the continuing proper delivery of absorbed dose by radiation therapy machines. The report also consid- era the salient features of exposure and absorbed-dose measurement that relate them to the national radiation standards and includes a discussion of the uncerfainty in the delivery of absorbed dose. The Council has noted the adoption by the 15th General Conference of Weights and Measures of special names for some units of the Systlrme International d'Unit& (SI) used in the field of ionizing radiation. The gray (symbol Gy) has been adopted as the special name for the SI unit of absorbed dose, absorbed dose index, kern, and specific energy imparted. The becquerel (symbol Bq) has been adopted as the special name for the SI unit of activity (of a radionuclide). One gray equals one joule per kilo- and one becquerel is equal to one second to the power of minus one. Since the tramition from the special units currently employed-rad and curie-to the new special names is expected to take some time, the Council has determined to continue, for the time being, the use of rad and curie. To convert from one set of units to the other, the following relationships pertain: 1r ad = 0.01 J kg-' = 0.01 Gy 1 curie = 3.7 x 101° s-I = 3.7 x 10" Bq (exactly). Serving on Scientific Committee 26 on High Energy X-Ray Dosi- metry during the preparation of this report were: m Robert J. Shalek, Chainnun Physica Department University of Texaa System Canmr Center M.D.A nderson Hospital aed Tumor InatitUte Houston. Texas Members Peter R Almond John S. Laughlin Physia Department Medical Phuaja bp&ment University of Teurs System Cancer Center Memorisl Sloan Ketimbg Cancer M.D. Anderson Hospital and Tumor Mtute Center Houston, Texas New York, New York John R Cameron Rabertbwbger Department of Radiology and Fh'- Radiation Physica Division University of Wisconsin National Bureau of Staodards Washington, D.C. h o l dF eldman Robert J. Schulz Methodist Medical Center of Illinois Department of Radiology Peoria. nlinois Yale University School of Medicine Lawrence H. Lanzl New Haven, Connecticut Department of Therapeutic Radiology ~~sh-~yterian-LSutk.e' s Medical Center Chicago, Illinois J. Garret&H olt Ralph B. Worsnop Medical Physics Department Bay h aM edical F'hysja, Inc. Memorial Sloan Kettering Cancer Center WoocI.de, California New York, New York Peter Wootton Department of Radiology University of Wasbagton Hoepita1 Seattle, Washington NCRP thmt~ht-Condsntine J. Mslcbk~e The Council wishes to express its appreciation to the members and consultants of the Committee for the time and effort devoted to the preparation of this report. Warren K. Sinclair President, NCRP Bethesda, Maryland March 1,1981 Contents Preface ................................................ iii . 1 Introduction ..................................... 1 1.1Purpose ............................................ 2 1.2 Discussion of Errors ................................. 2 . 1.3 Comprehensive References ........................... 3 2 Basic Parameters of Photon Beams and Principles of Dosimetry ............................................. 4 2.1 Interaction of Photons with Matter ................... 4 2.2 Energy of Photon Beams (Radiation Quality) .......... 4 2.3 Principles for the Determination of Absorbed Dose Pro- duced by Photons ................................... 5 2.3.1 Perspective ..................................... 5 2.3.2 Radiation Quantities and Units ................... 6 2.3.3 Relationship Between Exposure and Absorbed Dose 11 2.3.4 Free-Air Chambers .............................. 12 2.3.5 Bragg-Gray Cavity Ionization Chambers .......... 13 2.3.6 Exposure-Calibrated Cavity-Chamber Method for Photons of Energy Greater than 0.6 MeV .......... 14 2.3.7 Calorimetric Method ............................ 19 . 2.3.8 Chemical Methods .............................. 20 3 . National Radiation Standards ......................... 22 4 Secondary Standards ................................. 23 4.1 Instruments for Secondary Standards ................. 23 4.2 Specification 3f Beam Quality ........................ 24 4.3 Methods of Calibrating Secondary Standards on Field Instruments ........................................ 24 4.4 Tests of Field Instruments to be Performed By Calibration . Laboratories ........................................ 25 5 Field Inetruments ..................................... 26 5.1 Types of Field Instruments ........................... 27 5.1.1The Condenser Chamber with Separate Elec- trometer ....................................... 27 5.1.2 Ionization Chamber Connected by Cable to a Null- Reading Electrometer .......................... 27 v 5.1.3 Ionization Chamber Connected by Cable to a Feed- back Electrometer .............................. 28 5.2 Frequency of Calibration of Field Instruments .......... 28 5.3 Constancy of Field Instruments ....................... 29 5.3.1 Constancy Tests with Radioactive Sources ........ 29 5.3.2 Constancy Tests by Intercomparison of Chambers in Simultaneous or Alternate Irradiations ............ 30 5.3.3 Constancy Test of Sensitivity of Electrometer ..... 30 5.3.4 Record Keeping Related to Calibration Instruments 31 5.3.5 Number of Instruments Available and Continuity of Calibration When Instruments are not Available ... 31 5.4 Linearity of Response ................................ 31 5.5 Corrections for Temperature and Atmospheric Pressure . 32 5.6 Electrical Leakage, Spurious Ionization ............... 34 5.7 Stem Effects ....................................... 35 5.8 Energy Response of Chambers and Chamber Wall Thickness ........................................ 35 6.9 Effects of Dose Rate ................................ 38 5.10 Error of Initial Readings ............................ 41 5.11 Electrometer Setting Prior to Mearmrement With Con- denser Ionization Chamber .......................... 41 5.12 Microwave Interferences ............................ 41 5.13 Storage of Field Instruments ........................ 42 5.14 Transport of Field Instnunents ...................... 42 . 6 Commission, Calibration, and Other Measurements on Radiation Therapy Machines .......................... 43 6.1 General Safety ...................................... 43 6.2 Mechanical, Electrical and Optical Features ............ 43 6.2.1 Mechanical and Electrical Safety for Patient ....... 43 6.2.2 Secondary Exposure Limitation .................. 44 68.3Alignment of the Therapy Beam, Localizing Light and Collimator Axis ............................. 44 6.2.4 Assurance of Centering of Isocentric Units ......... 48 6.2.5 Alignment of Auxiliary Lights and Pointers ........ 48 6.2.6 Fidelity of Distance Indicators ................... 48 68.7 Stability of Treatment Couch During Treatment ... 49 6.3 Beam and Machine Characteristics Relating to the Cali- bration of Radiation Therapy Machines ............... 49 6.3.1 Introduction .................................... 49 6.3.2 Specification of the Energy of Therapy Beams ..... 49 6.3.3 Definition of Field Size .......................... 51 6.3.4 Beam Uniformity ............................... 52 6.3.5 Field Size Dependence .......................... 52 6.3.6 Apparent Position of the Source ................ 53 6.3.7 End Errors ................................... 54 6.3.8 Integrity of a Beam Monitor System ............. 56 6.3.9 Dependence of Absorbed-Dose Rate on Machine Orientation ................................... 58 6.3.10 Attenuation by Block Support Tray ............ 58 6.3.11 Electron Contamination ....................... 58 6.3.12 Neutron Contamination ....................... 59 6.4 Calibration ........................................ 60 6.4.1 Introduction .................................... 60 6.43 Calibration of X-Ray Machines and Radionuclide Ir- radiators in Air ................................. 60 6.4.3 Calibration of X-Ray Machines of Peak Energy From 2 to 50 MeV, and 13'Cs and wCo Irradiators in Water 64 6.4.4 Summary of Recommendations for Calibration Media ......................................... 67 6.5 Constancy Checks ................................... 68 6.5.1 Constancy Checks for X-Ray Machines of Peak En- ergy from 10 keV to 2 MeV ...................... 68 6.52 Constancy Checks for X-Ray Machines of Peak En- ergy from 2 to 50 MeV, and 13%s and @'Co Irra- diatom ......................................... 68 6.5.3 Constancy Checks Using Mailed Thermolurninescent Dosimeters ..................................... 69 6.6 Frequency of Calibration and Routine Checks of Operation .......................................... 69 6.6.1 Weekly Checks ................................. 69 6.69 Monthly Checks ................................ 70 6.6.3 Initial and Annual Checks ....................... 70 6.6.4 Recalibration of Field Instruments ................ 71 6.6.5 Independent Review ............................ 71 6.7 Absorbed-Dose Distributions ......................... 71 6.7.1 Expressions of Relative Absorbed Dose ........... 72 6.7.2 Methods of Measurement ........................ 74 6.7.3 Calculation of Useful Parameters from Calibrations at Recommended Calibration Depths for Gamma Rays from InCs and @'Coa nd X Rays of Peak Energy . Equal to or Greater than 2 MeV .................. 75 7 Uncertainty in Delivery of Absorbed Dose ............. 77 7.1 Typee of Uncertainty .............................. 77 7.2 Description of the Model ............................. 78 . 7.2.1 Step 1 Standardization of the NBS Beam ......... 79 7.2.2 Step 2 . Calibration of the Secondary Instrument ... 80 . 7.2.3 Step 3 Calibration of the Field Instrument at the Regional Calibration Laboratory ................. 80 . 7.2.4Step 4 Calibration of the Treatment Beam in the Hospital ..................... ... ... .... .... 81 . 7.2.5 Step 5 Delivery of Dose to the Tisrme Phantom ... 81 . 7.2.6 Step 0 The Physical Constants .................. 81 7.3 Overall Uncertainty ............................... 82 APPENDIX A Definitions ................................ 84 References ............................................... 88 TheNCRP ................................................ 96 NCRP Publications ........................................ 103 Index .................................................. 115 1. Introduction There is evidence suggesting that differences as small as 5 to 10 percent in the absorbed dose delivered in radiation therapy may result in differences in the local control of tumors for some types of treatment (Shukovsky, 1970). Likewise, there are a number of unpublished ex- periences where clinicians have been able to observe differences in clinical response resulting from absorbed doses differing by 7 percent A study of the absorbed dose delivered compared to that prescribed for radiation therapy in 174 institutions in the United States during the period 1968 to 1976 indicated that, for about 88 percent of the treatment machines reviewed, the calculated fulfillment of tumor dose prescription was within f5 percent of that intended. For those ma- chines falling outside f5 percent, the range in the ratio of delivered dose to the prescribed dose for uncomplicated treatments was 1.23 to 0.75 (Shalek et al., 1976). Errors were due to faults in radiation measurement, machine function, or calculation; at almost every insti- tution small systematic errors, often compensatory, were found. Some of the types of problems encountered are considered in this report. It is to be emphasized that the comment. in this paragraph and this report relate to the systems of dose measurements and calculation, and do not include accidental errors such as those introduced by patient movement during treatment, arithmetic errors, or misetting the controls of a treatment machine. Whenever possible, the physical uncertainties involved in fulfilling dose prescription should be substantially less than dose deviations that are of importance clinically. Many steps, each subject to error, are taken in linking the national radiation standards to the time or monitor setting of a radiation-therapy machine for the treatment of a patient in the fulfiUment of a radiation dose prescription. Many of these steps will be d d b e d together with procedures considered to be the best available for them at this time. This report is directed to the individuals responsible for dose determination at institutions adminkwing radia- tion therapy. Usually these individuals are medical physicists. At the outset, it must be stated that there are uncertainties in the methods recommended for the measurement of absorbed dose from high-energy x rays with ionization chambers. The transition from the calibration energy of ionization chambers Co@ '( gamma rays or 2-MV 1 x rays) to x rays of higher energy is more complex than previously thought. However, the values of CAg iven in Table 6 agree with those in common use since 1964. Any changes in these values likely to take place probably will be in the range of 0 to 3 percent depending upon the x-ray energy and the type of ionization chamber involved. Further discussion of this subject appears in Section 2.3. The recommendations of this report are preceded by a discussion of the pertinent characteristics of radiation beams, and of the salient features of radiation-exposure and absorbed-dose measurement that relate to the establishment of national radiation standards and the dissemination of the radiation units determined by these standards. References to more complete discussions are also given. The purpose of this report is to describe the systems and method6 by which the absorbed dose to a homogeneous phantom simulating a patient undergoing radiation therapy may be determined and related to national radiation standards. The radiations under consideration are x rays with peak energies in the range h m 1 0 keV to 50 MeV and gamma rays h m r adionuclide therapy units. The rationale, the care and use of instruments, and the measurements required for radiation- therapy machines are dkussed. Discussion is limited to a single treatment field in a water or tissue phantom. Treatment planning is not discussed. It is hoped that the implementation of the recommen- dations, which are weighted by "ahall" and "should,"' will result in an improvhment in the fulfillment of dose prescription in radiation ther- apy. The report does not addresa machine speci.6cations for manufac- turers or for the design of facilities as required by state and federal regulations or covered in other NCRP reporta Therefore, mechanical, electrical, and radiation specifications relating to the general operation of the machine and to the safety of the patient, the operator and the public are sjwifically omitted. Some measuring instruments which are commonly used in the field may be of marginal performance when compared to the clinical needs, ' See Appendi. A De6nitiora.