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Monte Carlo calculations of correction factors for plane‐parallel ionization chambers in clinical electron dosimetry
Author(s) -
Araki Fujio
Publication year - 2008
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 - 10.1118/1.2968102
Subject(s) - dosimetry , monte carlo method , ionization , electron , physics , computational physics , ionization chamber , percentage depth dose curve , atomic physics , plane (geometry) , cathode ray , optics , nuclear physics , nuclear medicine , ion , geometry , mathematics , medicine , statistics , quantum mechanics
Recent standard dosimetry protocols recommend that plane‐parallel ionization chambers be used in the measurements of depth‐dose distributions or the calibration of low‐energy electron beams with beam qualityR 50 < 4 g ∕ cm 2 . In electron dosimetry protocols with the plane‐parallel chambers, the wall correction factor, P wall , in water is assumed to be unity and the replacement correction factor, P repl , is taken to be unity for well‐guarded plane‐parallel chambers, at all measurement depths. This study calculated P wall and P repl for NACP‐02, Markus, and Roos plane‐parallel chambers in clinical electron dosimetry using the EGSnrc Monte Carlo code system. The P wall values for the plane‐parallel chambers increased rapidly as a function of depth in water, especially at lower energy. The value around R 50 for NACP‐02 was about 10% greater than unity at 4 MeV . The effect was smaller for higher electron energies. Similarly, P repl values with depth increased drastically at the region with the steep dose gradient for lower energy. For Markus P repl departed more than 10% from unity close to R 50 due to the narrow guard ring width. P repl for NACP‐02 and Roos was close to unity in the plateau region of depth‐dose curves that includes a reference depth, d ref . It was also found that the ratio of the dose to water and the dose to the sensitive volume in the air cavity for the plane‐parallel chambers,D w ∕ [ D air ] pp , at d ref differs significantly from that assumed by electron dosimetry protocols.