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 = 0.6 R 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.