Abstract: By establishing female squatting posture BREP phantom based on Chinese reference female BREP phantom,this paper researched the effects of posture change to photon dose conversion coefficients. Using Monte Carlo methods, the organ dose conversion coefficients and the effective dose conversion coefficients of 21 monoenergetic photon beams from 0.01 to 10 MeV under six standard irradiation geometries were calculated. Furthermore,the differences were compared between squatting and standing postures. It is found that there were greater differences among the organ absorbed dose conversion coefficients in anterior-posterior,posterior-anterior,left and right lateral irradiation geometries. For example,the absorbed dose conversion coefficient of breast is 115.1% higher than standing position under posterior-anterior irradiation geometry at 0.03 MeV. However, the differences of the effective dose conversion coefficients are within 10% in most energy points. The differences in conversion coefficients are due to the position of arms,legs and the thickness changes in irradiation direction of rays for squatting position.

Key words: squatting posture, boundary representation phantom, Monte Carlo methods, dose conversion coefficients