Replacement correction factors for cylindrical ion chambers in electron beams

Purpose: In the TG-21 dosimetry protocol, for cylindrical chambers in electron beams the replacement correction factor P(repl) (or the product p(dis)p(cav) in the IAEA's notation), was conceptually separated into two components: the gradient correction (P(gr)) accounting for the effective point of measurement and the fluence correction (P(fl)) dealing with the change in the electron fluence spectrum. At the depth of maximum dose (d(max)), P(gr) is taken as 1. There are experimental data available at d(max) for the values of P(fl) (or P(repl)). In the TG-51 dosimetry protocol, the calibration is at the reference depth d(ref)=0.6R(50)-0.1 (cm) where P(gr) is required for cylindrical chambers and P(fl) is unknown and so the measured values at d(max) are used with the corresponding mean electron energy at d(ref). Monte Carlo simulations are employed in this study to investigate the replacement correction factors for cylindrical chambers in electron beams. Methods: Using previously established Monte Carlo calculation methods, the values of P(repl) and P(fl) 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 P(gr) as defined in the TG-51 dosimetry protocol are also calculated. Results: The calculated values of the fluence correction factors P(fl) 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 P(fl) for cylindrical chambers at d(ref) in electron beams is derived as a function of the chamber radius and the beam quality specifier R(50). Conclusions: The mean electron energy at depth is a good beam quality specifier for P(fl). Thus TG-51's adoption of P(fl) at d(max) with the same mean electron energy for use at d(ref) is proven to be accurate. The values of P(gr) 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. (C) 2009 American Association of Physicists in Medicine. [DOI: 10.1118/1.3213094]