Development of PC based Monte Carlo simulations for the calculation of scanner-specific normalized organ doses from CT.

This paper discusses the simulation of contemporary computed tomography (CT) scanners using Monte Carlo calculation methods to derice normalized organ doses, which enable hospital physicists to estimate typical organ and effective doses for CT examinations. The hardware used in a small PC-cluster at the Health Protection Agency (HPA) for these calculations is described. Investigations concerning optimization of software, including the radiation transport codes MCNP5 and MCNPX, and the Intel and PGI FORTRAN compilers, are presented in relation to results and calculation speed. Differences in apporach for modelling the X-ray source are described and their influences are analysed. Comparisons with previously published calculations at HPA from the early 1990s proved satisfactory for the purpose of quality assurance and are presented in terms of organ dose ratios for whole body exposure and differences in organ location. Influences on nomalized effective dose are discussed in relation to choice of cross section library, CT scanner technology (contemporary multi slice versus single slice), defintition for effective dose (1990 and 2007 versions) and anthropomorphic phantom coefficients presently being calculated at HPA, in order to facilitate improved dosimetry for contemporary CT practice