Absorbed dose values were determined under the reference conditions in a phantom irradiated by high-energy photon beams with quality varying between60Co gamma rays and 25 MV X-rays, using four commonly employed types of ionisation chamber. The ionisation chamber readings were converted to absorbed dose values applying the recent NACP, AAPM and SEFM Protocols and the revised HPA Code of Practice. The AAPM and SEFM Protocols gave consistent results for the four types of chamber whereas the NCAP Protocol should be adapted to take the differences in chamber wall material and chamber dimensions into account. Absorbed dose values determined with the standard chamber and procedure recommended in the HPA Code of Practice show good agreement, within 0.8, with absorbed dose values obtained using the AAPM and SEFM Protocols.
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