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Sci‐YIS Fri ‐ 02: Calculated P wall values in clinical photon beams
Author(s) -
Buckley L,
Rogers D
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 - 10.1118/1.2031024
Subject(s) - monte carlo method , dosimetry , photon , physics , ionization chamber , computational physics , absorbed dose , ionization , electron , photon energy , beam (structure) , optics , atomic physics , ion , nuclear physics , nuclear medicine , mathematics , radiation , statistics , quantum mechanics , medicine
Dosimetry of high‐energy photon beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. This study investigates the wall correction factor, P wall , in high‐energy photon beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. For cylindrical chambers, dosimetry protocols use an empirical formula to determine P wall despite high quality experimental evidence that there are problems with this formula. Wall corrections are not provided for parallel‐plate chambers in photon beams due to a lack of information available regarding the correction factors for these chambers. Monte Carlo calculations are carried out using the EGSnrc user‐code CSnrc to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. Calculations of the wall correction are performed for a variety of chambers at the reference depth in photon beams, using realistic beam spectra from clinical accelerators, ranging in nominal energy from60 Co to 24 MV. For cylindrical chambers, P wall values differ by as much as 0.8% from the predicted values. This discrepancy is used to resolve previous experimental results that pointed to problems with the P wall formalism. P wall values are also shown for parallel‐plate chambers in high‐energy photon beams and have corrections up to 2%. These data should allow parallel‐plate chambers to be used in photon as well as in electron beams.