The replacement correction factors for cylindrical chambers in high-energy photon beams.

The values of the replacement correction factors (P(repl), or in the IAEA's notation p(dis)p(cav)) used for cylindrical chambers in high-energy photon beams represent one of the most significant differences between the AAPM and the IAEA dosimetry protocols. In a previous study (Wang L L W and Rogers D W O 2008 Med. Phys. 35 1747-55), we found that the AAPM protocol adopted incorrect values of P(repl) for cylindrical chambers in photon beams. For a (60)Co beam, the calculated P(repl) value is 0.5% higher than the AAPM value and about 1% higher than the IAEA value. It was still not clear why the IAEA values, which are based on measurements by Johansson et al, are incorrect. In this study, EGSnrc Monte Carlo simulation codes are used to simulate Johansson et al's experimental procedures for determining P(repl) values. The simulation results agree well with the measurements if the chamber responses versus depth are normalized at d(max) as was apparently done in the experiments as it was believed that the chambers of different radii gave the same maximum reading at the respective d(max). However, if the chamber responses are not normalized, then the simulated experimental results lead to a result which agrees well with the P(repl) values calculated by the standard Monte Carlo methods. This demonstrates that the normalization procedure used in the experiments is incorrect as is based on an incorrect assumption, and thus the interpretation of Johansson et al's experimental values as P(repl) (p(dis)) in the IAEA TRS-398 Code of Practice is wrong. The values of P(repl) for cylindrical chambers of different radii in various high-energy photon beams are calculated and an empirical formula is given.