Monte Carlo calculations of correction factors for plane‐parallel ionization chambers in clinical electron dosimetry

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.