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Poster — Thur Eve — 21: Off‐axis dose perturbation effects in water in a 5 × 5 cm 2 18 MV photon beam for the PTW microLion and Exradin A1SL ionization chambers
Author(s) -
O'Grady K,
Davis S D,
Papaconstadopoulos P,
Seuntjens J
Publication year - 2014
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 - 10.1118/1.4894877
Subject(s) - ionization chamber , physics , monte carlo method , ionization , percentage depth dose curve , beam (structure) , atomic physics , dosimetry , electric field , linear particle accelerator , optics , nuclear medicine , ion , medicine , statistics , mathematics , quantum mechanics
A PTW microLion liquid ionization chamber and an Exradin A1SL air‐filled ionization chamber have been modeled using the egs_chamber user code of the EGSnrc system to determine their perturbation effects in water in a 5 × 5 cm 2 18 MV photon beam. A model of the Varian CL21EX linear accelerator was constructed using the BEAMnrc Monte Carlo code, and was validated by comparing measured PDDs and profiles from the microLion and A1SL chambers to calculated results that included chamber models. Measured PDDs for a 5 × 5 cm 2 field for the microLion chamber agreed with calculations to within 1.5% beyond a depth of 0.5 cm, and the A1SL PDDs agreed within 1.0% beyond 1.0 cm. Measured and calculated profiles at 10 cm depth agreed within 1.0% for both chambers inside the field, and within 4.0% near the field edge. Local percent differences increased up to 15% at 4 cm outside the field. The ratio of dose to water in the absence of the chamber relative to dose in the chamber's active volume as a function of off‐axis distance was calculated using the egs_chamber correlated sampling technique. The dose ratio was nearly constant inside the field and consistent with the stopping power ratios of water to detector material, but varied up to 3.3% near the field edge and 5.2% at 4 cm outside the field. Once these perturbation effects are fully characterized for more field sizes and detectors, they could be applied to clinical water tank measurements for improved dosimetric accuracy.

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