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

The replacement correction factor (P ) in ion chamber dosimetry accounts for the effects of the medium being replaced by the air cavity of the chamber. In TG-21, P was conceptually separated into two components: fluence correction, P , and gradient correction, P . In TG-51, for electron beams, the calibration is at d where P is required for cylindrical chambers and P is unknown and assumed to be the same as that for a beam having the same mean electron energy at d . For cylindrical chambers in high-energy photon beams, P also represents a major uncertainty in current dosimetry protocols. In this study, P 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 ; and TG-51's adoption of P at d with the same mean electron energy for use at d is proven to be accurate. For Farmer chambers in photon beams, there is essentially no beam quality dependence for P values. In a Co photon beam, the calculated P is about 0.4-0.6% higher than the TG-21 value, indicating TG-21 (and TG-51) used incorrect values of P for cylindrical chambers.