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Calibration of the Absorbed Dose Produced in Water by Betatron Electrons with the Benzoic-Acid Dosimeter

Calibration of the Absorbed Dose Produced in Water by Betatron Electrons with the Benzoic-Acid... Calibration of the Absorbed Dose Produced in Water by Betatron Electrons with the Benzoic-Acid Dosimeter 1 Nathaniel F. Barr , Ph.D. , Mary B. Stark , B.S. and John S. Laughlin , Ph.D. Massachusetts General Hospital Boston 14, Mass. Excerpt Monitoring of betatron electron beams used for therapy in the United States during the past decade has most frequently been done with a transmission ionization chamber, the output of which is calibrated periodically in terms of the reading of a Victoreen ionization chamber surrounded by sufficient additional plastic material to obtain the maximum response (1, 2). These monitor measurements may be converted to absorbed dose by calculation (3) or by calibration with the absorbed dose calorimeter (4). This conversion is more conveniently accomplished through the use of a suitable chemical dosimeter. Such an approach is also more direct than the use of calculated conversion factors, which necessarily involve major assumptions. In the experiments to be described here the benzoic-acid dosimeter has been used to determine the absorbed dose rate produced by betatron electrons at the depth of relative maximum dose in water. These measurements have been related to the conventional and arbitrary specification of betatron electron exposure in terms of the reading of a Victoreen thimble chamber inserted in a polystyrene block. Chemical yield and Victoreen chamber readings have been obtained for all field sizes and energies (6 to 20 Mev) used in therapeutic applications of the betatron electron beam. Replicate determinations of energy absorbed with the Fricke ferrous sulfate dosimeter for all combinations of field sizes and energies would be very time-consuming at therapeutic dose rates. Achemical dosimeter in which benzoic acid is oxidized to salicylic acid is at least ten times more sensitive than the Fricke dosimeter (5), and its use permits considerable saving of time. Since the absolute yield of this dosimeter for electrons is not known, it was calibrated in terms of the Fricke dosimeter for 10- and 20-Mev electrons, on the basis of 15.5 ferricions per 100 e.v. absorbed for the absolute yield of the Fricke dosimeter. The Fricke dosimeter solution was irradiated in the identical geometry, as described below, and in the same containers as the benzoic acid dosimeter. The irradiation field size was 6 × 8 cm. for both 10- and 20-Mev electrons. The ferrous sulfate solutions were irradiated to 5,200 rads and the benzoic acid solutions to 250 rads. The irradiations were monitored by a special transmission ionization chamber and integrating circuit of very good reproducibility (2). Three samples of each chemical dosimeter were irradiated, in alternating sequence, at each energy. A direct calorimetric determination of this yield for a variety of electron energies is now in progress in our laboratory. Experimental Twice recrystallized reagent grade benzoic acid was used throughout. The dosimeter solution consisted of an aqueous solution 1 millimolar in benzoic acid and 10 millimolar in sodium hydroxide. A large cylindrical tank of water 20 cm. deep and 15 cm. in diameter was irradiated. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiology Radiological Society of North America, Inc.

Calibration of the Absorbed Dose Produced in Water by Betatron Electrons with the Benzoic-Acid Dosimeter

Barr, Nathaniel F.; Stark, Mary B.; Laughlin, John S.
Radiology , Volume 78 (4): 625 – Apr 1, 1962
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References (6)

Publisher
Radiological Society of North America, Inc.
Copyright
Copyright © 1962 by Radiological Society of North America
ISSN
1527-1315
eISSN
0033-8419
DOI
10.1148/78.4.625
pmid
13865188
Publisher site
See Article on Publisher Site

Abstract

Calibration of the Absorbed Dose Produced in Water by Betatron Electrons with the Benzoic-Acid Dosimeter 1 Nathaniel F. Barr , Ph.D. , Mary B. Stark , B.S. and John S. Laughlin , Ph.D. Massachusetts General Hospital Boston 14, Mass. Excerpt Monitoring of betatron electron beams used for therapy in the United States during the past decade has most frequently been done with a transmission ionization chamber, the output of which is calibrated periodically in terms of the reading of a Victoreen ionization chamber surrounded by sufficient additional plastic material to obtain the maximum response (1, 2). These monitor measurements may be converted to absorbed dose by calculation (3) or by calibration with the absorbed dose calorimeter (4). This conversion is more conveniently accomplished through the use of a suitable chemical dosimeter. Such an approach is also more direct than the use of calculated conversion factors, which necessarily involve major assumptions. In the experiments to be described here the benzoic-acid dosimeter has been used to determine the absorbed dose rate produced by betatron electrons at the depth of relative maximum dose in water. These measurements have been related to the conventional and arbitrary specification of betatron electron exposure in terms of the reading of a Victoreen thimble chamber inserted in a polystyrene block. Chemical yield and Victoreen chamber readings have been obtained for all field sizes and energies (6 to 20 Mev) used in therapeutic applications of the betatron electron beam. Replicate determinations of energy absorbed with the Fricke ferrous sulfate dosimeter for all combinations of field sizes and energies would be very time-consuming at therapeutic dose rates. Achemical dosimeter in which benzoic acid is oxidized to salicylic acid is at least ten times more sensitive than the Fricke dosimeter (5), and its use permits considerable saving of time. Since the absolute yield of this dosimeter for electrons is not known, it was calibrated in terms of the Fricke dosimeter for 10- and 20-Mev electrons, on the basis of 15.5 ferricions per 100 e.v. absorbed for the absolute yield of the Fricke dosimeter. The Fricke dosimeter solution was irradiated in the identical geometry, as described below, and in the same containers as the benzoic acid dosimeter. The irradiation field size was 6 × 8 cm. for both 10- and 20-Mev electrons. The ferrous sulfate solutions were irradiated to 5,200 rads and the benzoic acid solutions to 250 rads. The irradiations were monitored by a special transmission ionization chamber and integrating circuit of very good reproducibility (2). Three samples of each chemical dosimeter were irradiated, in alternating sequence, at each energy. A direct calorimetric determination of this yield for a variety of electron energies is now in progress in our laboratory. Experimental Twice recrystallized reagent grade benzoic acid was used throughout. The dosimeter solution consisted of an aqueous solution 1 millimolar in benzoic acid and 10 millimolar in sodium hydroxide. A large cylindrical tank of water 20 cm. deep and 15 cm. in diameter was irradiated.

Journal

Radiology – Radiological Society of North America, Inc.

Published: Apr 1, 1962

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