Replacement correction factors for cylindrical ion chambers in electron beams

Abstract

In the TG-21 dosimetry protocol, for cylindrical chambers in electron beams the replacement correction factor Prepl (or the product PdisPcav in the IAEA's notation), was conceptually separated into two components: the gradient correction (Pgr) accounting for the effective point of measurement and the fluence correction (Pfl) dealing with the change in the electron fluence spectrum. At the depth of maximum dose (dmax), Pgr is taken as 1. There are experimental data available at dmax for the values of Pfl (or Prepl). In the TG-51 dosimetry protocol, the calibration is at the reference depth dref=0.6R50-0.1 (cm) where Pgr is required for cylindrical chambers and Pfl is unknown and so the measured values at dmax are used with the corresponding mean electron energy at dref. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for cylindrical chambers in electron beams. Using previously established Monte Carlo calculation methods, the values of Prepl and Pfl are calculated with high statistical precision (<0.1%) for cylindrical cavities of a variety of diameters and lengths in a water phantom irradiated by various electron beams. The values of Pgr as defined in the TG-51 dosimetry protocol are also calculated. The calculated values of the fluence correction factors Pfl are in good agreement with the measured values when the wall correction factors are taken into account for the plane-parallel chambers used in the measurements. An empirical formula for Pfl for cylindrical chambers at dref in electron beams is derived as a function of the chamber radius and the beam quality specifier R50. The mean electron energy at depth is a good beam quality specifier for Pfl. Thus TG-51's adoption of Pfl at dmax with the same mean electron energy for use at dref is proven to be accurate. The values of Pgr for a Farmer-type chamber as defined in the TG-51 dosimetry protocol may be wrong by 0.3% for high-energy electron beams and by more than 1% for low-energy electron beams.