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The effect of ambient pressure on well chamber response: Monte Carlo calculated results for the HDR 1000 Plus
Author(s) -
Bohm Tim D.,
Griffin Sheridan L.,
DeLuca Paul M.,
DeWerd Larry A.
Publication year - 2005
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1884905
Subject(s) - ionization chamber , monte carlo method , kerma , photon , physics , elevation (ballistics) , photon energy , ionization , computational physics , dosimetry , nuclear medicine , nuclear physics , optics , mathematics , ion , medicine , statistics , quantum mechanics , astronomy
The determination of the air kerma strength of a brachytherapy seed is necessary for effective treatment planning. Well ionization chambers are used on site at therapy clinics to determine the air kerma strength of seeds. In this work, the response of the Standard Imaging HDR 1000 Plus well chamber to ambient pressure is examined using Monte Carlo calculations. The experimental work examining the response of this chamber as well as other chambers is presented in a companion paper. The Monte Carlo results show that for low‐energy photon sources, the application of the standard temperature pressureP TPcorrection factor produces an over‐response at the reduced air densities/pressures corresponding to high elevations. With photon sources of 20 to 40 keV, the normalizedP TPcorrected chamber response is as much as 10% to 20% over unity for air densities/pressures corresponding to an elevation of 3048 m (10000 ft) above sea level. At air densities corresponding to an elevation of 1524 m (5000 ft), the normalizedP TP ‐corrected chamber response is 5% to 10% over unity for these photon sources. With higher‐energy photon sources ( > 100 keV ) , the normalizedP TPcorrected chamber response is near unity. For low‐energy β sources of 0.25 to 0.50 MeV, the normalizedP TP ‐corrected chamber response is as much as 4% to 12% over unity for air densities/pressures corresponding to an elevation of 3048 m (10000 ft) above sea level. Higher‐energy β sources ( > 0.75 MeV ) have a normalizedP TPcorrected chamber response near unity. Comparing calculated and measured chamber responses for common Pd103 ‐ and I125 ‐based brachytherapy seeds show agreement to within 2.7% and 1.9%, respectively. Comparing MCNP calculated chamber responses with EGS nrc calculated chamber responses show agreement to within 3.1% at photon energies of 20 to 40 keV. We conclude that Monte Carlo transport calculations accurately model the response of this well chamber. Further, applying the standardP TPcorrection factor for this well chamber is insufficient in accounting for the change in chamber response with air pressure for low‐energy ( < 100 keV ) photon and low‐energy( < 0.75 MeV ) β sources.