Sci-Fri PM: Planning-10: The replacement correction factors for cylindrical chambers in megavoltage beams

The replacement correction factor (P_repl ) in ion chamber dosimetry accounts for the effects of the medium being replaced by the air cavity of the chamber. In TG-21, P_repl was conceptually separated into two components: fluence correction, P_fl , and gradient correction, P_gr . In TG-51, for electron beams, the calibration is at d_ref where P_gr is required for cylindrical chambers and P_fl is unknown and assumed to be the same as that for a beam having the same mean electron energy at d_max . For cylindrical chambers in high-energy photon beams, P_repl also represents a major uncertainty in current dosimetry protocols. In this study, P_repl is calculated with high precision (<0.1%) by the Monte Carlo method as the ratio of the dose in a phantom to the dose scored in water-walled cylindrical cavities of various radii (with the center of the cavity being the point of measurement) in both high energy photon and electron beams. It is found that, for electron beams, the mean electron energy at depth is a good beam quality specifier for P_fl ; and 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. For Farmer chambers in photon beams, there is essentially no beam quality dependence for P_repl values. In a Co photon beam, the calculated P_repl is about 0.4-0.6% higher than the TG-21 value, indicating TG-21 (and TG-51) used incorrect values of P_repl for cylindrical chambers.