The effect of ambient pressure on well chamber response: Monte Carlo calculated results for the HDR 1000 Plus
- University of Wisconsin-Madison, Department of Medical Physics, Madison, Wisconsin 53706 (United States)
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 pressure P{sub TP} correction factor produces an over-response at the reduced air densities/pressures corresponding to high elevations. With photon sources of 20 to 40 keV, the normalized P{sub TP} corrected 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 normalized P{sub TP}-corrected chamber response is 5% to 10% over unity for these photon sources. With higher-energy photon sources (>100 keV), the normalized P{sub TP} corrected chamber response is near unity. For low-energy {beta} sources of 0.25 to 0.50 MeV, the normalized P{sub 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 {beta} sources (>0.75 MeV) have a normalized P{sub TP} corrected chamber response near unity. Comparing calculated and measured chamber responses for common {sup 103}Pd- and {sup 125}I-based brachytherapy seeds show agreement to within 2.7% and 1.9%, respectively. Comparing MCNP calculated chamber responses with EGSnrc 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 standard P{sub TP} correction 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){beta} sources.
- OSTI ID:
- 20634651
- Journal Information:
- Medical Physics, Vol. 32, Issue 4; Other Information: DOI: 10.1118/1.1884905; (c) 2005 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
Similar Records
Air-kerma strength determination of a new directional {sup 103}Pd source
SU-E-T-552: Monte Carlo Calculation of Correction Factors for a Free-Air Ionization Chamber in Support of a National Air-Kerma Standard for Electronic Brachytherapy