Equivalent dose to skeletal soft tissues based on μCT images of trabecular bone for external exposure to photons

The purpose of this study is to introduce μCT images of human spongiosa into the skeleton of the MAX06 (Male Adult voXel) phantom to calculate equivalent doses to the red bone marrow (RBM) and to the bone surface cells (BSC) for external exposure to photons directly in the marrow cavities of trabecular bone by Monte Carlo methods. The 8SP cluster method employs a cluster (= parallelepiped) of 2 x 2 x 2 =8 micro matrices systematically and periodically (SP) throughout the spongiosa volume. The results show that RBM and BSC equivalent doses do not change if the cluster contains more than 8 micro matrices. Therefore, using a cluster with 8 micro matrices represents the minimum coherent skeletal microstructure information necessary to simulate radiation particle transport in human spongiosa. Compared to earlier methods used for the calculation of the RBM and BSC equivalent doses from external exposure to photons, the 8 SP cluster method produces generally smaller values. This happens because earlier methods use homogeneous skeletons, in which the cortical bone matrix becomes contributor to the RBM and BSC equivalent dose instead of functioning as shielding for the underlying spongiosa, and also because they use the kerma approximation which ignores the escape of secondary electrons from the spongiosa volume for higher energies. (author)