Skeletal dosimetry for external exposure to photons based on microCT images of spongiosa from different bone sites.

Micro computed tomography (microCT) images of human spongiosa have recently been used for skeletal dosimetry with respect to external exposure to photon radiation. In this previous investigation, the calculation of equivalent dose to the red bone marrow (RBM) and to the bone surface cells (BSC) was based on five different clusters of micro matrices derived from microCT images of vertebrae, and the BSC equivalent dose for 10 microm thickness of the BSC layer was determined using an extrapolation method. The purpose of this study is to extend the earlier investigation by using microCT images from eight different bone sites and by introducing an algorithm for the direct calculation of the BSC equivalent dose with sub-micro voxel resolution. The results show that for given trabecular bone volume fractions (TBVFs) the whole-body RBM equivalent dose does not depend on bone site-specific properties or imaging parameters. However, this study demonstrates that apart from the TBVF and the BSC layer thickness, the BSC equivalent dose additionally depends on a so-called trabecular bone structure (TBS) effect, i.e. that the contribution of photo-electrons released in trabecular bone to the BSC equivalent dose also depends on the bone site-specific structure of the trabeculae. For a given bone site, the TBS effect is also a function of the thickness of the BSC layer, and it could be shown that this effect would disappear almost completely, should the BSC layer thickness be raised from 10 to 50 microm, according to new radiobiological findings.