Absorbed fractions for alpha-particles in tissues of trabecular bone: considerations of marrow cellularity within the...

Alpha-particles are of current interest in radionuclide therapy due to their short range and high rates of energy transfer to target tissues. Published values of alpha-particle absorbed fraction phi in the skeletal tissues, as needed for patient-specific dosimetry under the MIRD schema, do not generally account for its variation with particle energy or skeletal site. Furthermore, variations in alpha-particle absorbed fraction with marrow cellularity have yet to be fully considered.

In this study, a 3-dimensional (3D) chord-based radiation transport model (or 3D-CBIST) is presented, which combines (a) chord-based techniques for tracking alpha-particles across bone trabeculae, endosteum, and marrow cavities and (b) a spatial model of the marrow tissues that explicitly considers the presence of marrow adipocytes. Chord-length distributions are taken from a 44-y male subject (ICRP [International Commission on Radiological Protection] Reference Male) and are identical to those used currently for clinical dose estimates for beta-particle emitters.

Values of phi(active marrow<--active marrow) given by the 3D-CBIST model are shown to be considerably lower than phi = 1.0 assumed under the ICRP Publication 30 and 2003 Eckerman bone models. For example, values of absorbed fraction for the self-dose to active bone marrow in the ribs, cervical vertebra, and parietal bone are 0.81, 0.80, and 0.55 for 6-MeV alpha-particles and are 0.74, 0.72, and 0.43 for 9-MeV alpha-particles, where each is evaluated at ICRP reference cellularities in the 3D-CBIST model (72%, 72%, and 42%, respectively, at age 25 y).

Improvements in patient-specific dosimetry of skeletal tissues require explicit consideration of not only changes in target mass with variable patient marrow cellularity (i.e., active marrow) but also corresponding changes in values of the absorbed fraction. The data given in this study provide a more-firm basis for application of the MIRD schema to patient-specific dosimetry for newly developing therapies using alpha-particle emitters.