Implementation of a microdosimetric model for radioimmunotherapeutic alpha emitters

A microdosimetric model for alpha-particle-emitting radiolabeled antibodies, based on an analytic method, was developed to be used for in vitro studies. The model took into consideration cell radii distributions or distributions of activity bound to cells, and calculated the single- and multihit distributions of specific energy within the target. The mean absorbed dose could then be derived from the specific energy spectra. The mean number of hits, the probability that no particle crossed the target, and the average lineal energy transfer at which the energy is deposited were also calculated. Many in vitro geometric configurations of cells (single cell, cellular monolayer, and cellular clusters) and many different distributions of radioactive sources observed in experiments (distribution on the cell surface or within the extracellular volume) could be modeled. To verify the implementation of our algorithm, a comparison was carried out for different sources and target configurations between our model and a general Monte Carlo code (MCNPX). A positive agreement was observed between the two approaches. By using the proposed model, computation speed was greatly improved, as compared with the Monte-Carlo approach. An example of the impact of some parameters (cell radii and activity distributions) on the dosimetric results is also given in this paper.