RESUMEN
Objective To study the difgerence between the IAEA code of practice TRS-Z77 and TRS-398 in the determination of the absorbed dose to water for high-energy photon beams using several cylindrical chambers. Methods For 6 different types of cylindrical chambers,the calibration factors ND,W,Q0 in terms of absorbed dose to water were calculated from the air exposure calibration factors N,,and were compared with the ND,W,Q0 measured in European standard laboratory. Accurate measurements were performed in Varian 6 MV photon beam using 6 cylindrical chambers according to TRS-277 and TRS-398.The beam quality correction factors kQ.Q0 as well as the water absorbed doses were compared.Results For the set of chambers,the difference between ND,W,Q0 computed from Nx and ND.-.Q0 obtained in European standard laboratory was 0.13％ ～ 1.30％.The difference of beam quality correction factors for TRS-277 and TRS-398 was 0.09％ ～0.45％.The distinction of the water absorbed doses obtained according to the two different protocols was 0.27％ ～ 1.40％,and was primarily due to their different calibration factors.Conclusions The discrepancy in absorbed doses determined according to two protocols using different cylindrical chambers is clinically acceptable.However,TRS-398 allows a more convenient localization of chambers,provides a more simple formulation,and offers the reduced uncertainty in the dosimetry of radiotherapy beams.