Photon SAF calculation based on the Chinese mathematical phantom and comparison with the ORNL phantoms

The Chinese mathematical phantom (CMP) is a stylized human body model developed based on the methods of Oak Ridge National Laboratory (ORNL) mathematical phantom series (OMPS), and data from Reference Asian Man and Chinese Reference Man. It is constructed for radiation dose estimation for Mongolians, whose anatomical parameters are different from those of Caucasians to some extent. Specific absorbed fractions (SAF) are useful quantities for the primary estimation of internal radiation dose. In this paper, a general Monte Carlo code, Monte Carlo N-Particle Code (MCNP) is used to transport particles and calculate SAF. A new variance reduction technique, called the "pointing probability with force collision" method, is implemented into MCNP to reduce the calculation uncertainty, especially for a small-volume target organ. Finally, SAF data for all 31 organs of both sexes of CMP are calculated. A comparison between SAF based on male phantoms of CMP and OMPS demonstrates that the differences apparently exist, and more than 80% of SAF data based on CMP are larger than that of OMPS. However, the differences are acceptable (the differences are above one order of magnitude only in less than 3% of situations) considering the differences in physique. Furthermore, trends in the SAF with increasing photon energy based on the two phantoms agree well. This model complements existing phantoms of different age, sex and ethnicity.