Dosimetry of X-Ray and Gamma-Ray Beams for Radiation Therapy in the Energy Range 10 keV to 50 MeV

123 Pages·1981·5.205 MB·English

by  NCRP

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
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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.