Response function for computing absorbed dose to skeletal tissues from photon irradiation.

The calculation of absorbed dose in skeletal tissues at radiogenic risk has been a difficult problem because the relevant structures can not be represented in conventional geometric terms nor can they be visualized in the tomographic image data used to define the computational models of the human body. The active marrow, the tissue of concern in leukemia induction, is present within the spongiosa regions of trabecular bone while the osteoprogenitor cells at risk for bone cancer induction are considered to be within the soft tissues adjacent to the mineral surfaces. The International Commission on Radiological Protection (ICRP) recommends averaging the absorbed energy over the active marrow within the spongiosa and over the soft tissues within 10 μm of the mineral surface for leukemia and bone cancer induction, respectively. In its forthcoming recommendation it is expected that the latter guidance will be changed to include soft tissues within 50 μm of the mineral surfaces. To address the computational problems, the skeleton of the proposed ICRP reference computational phantom has been subdivided to identify those voxels associated with cortical shell, spongiosa, and the medullary cavity of the long bones. It is further proposed that in Monte Carlo calculations with the phantoms that the energy deposition in the target tissues be derived as the product of the particle fluence in the skeletal subdivision and fluence-to-dose response functions. This paper outlines the development of such response functions for photons