Monte Carlo microdosimetry of 211at in multi-species thyroid follicle models: AlfaMC versus MCNPX and Geant4.

BACKGROUND: Radionuclide therapy is an increasingly used treatment modality for patients with disseminated cancer diseases. Tumor-binding radiopharmaceuticals labelled with the alpha-particle emitter astatine-211 (211At) are of interest to enable high radiation absorbed doses to small tumors. The alpha particles emitted by 211At will deposit all their energy in a short-range including neighboring cells, causing localized irradiation. Due to being part of group 17 the halogen family with iodine, free 211At accumulates in the thyroid gland, which is the main normal organ at risk. It is therefore of great importance to estimate the radiation absorbed dose accurately to the thyroid gland for free 211At. The aims of the work were to determine microdosimetric quantities for uniform and non-uniform 211At distributions within different models of the thyroid gland using the Monte Carlo (MC) codes AlfaMC and GEometry ANd Tracking (Geant4), and to compare the results with previously published data using Monte Carlo N-Particle eXtended (MCNPX). AlfaMC is customized for calculating microdosimetric quantities for alpha-particles. RESULTS: Both a single and a multiple thyroid follicle model designed for man, rat and mouse were used from previously published studies. Monte Carlo simulations and absorbed dose-calculations were performed for different distributions of 211At within the thyroid models. The mean specific energy, single-hit mean specific energy and single-hit specific energy distribution were calculated for follicle cell nuclei in models for each species. CONCLUSION: The overall agreement between AlfaMC and MCNPX results, and between AlfaMC and Geant4 results, was found to be within 5% and 2%, respectively. The intercomparison suggested an effective way for validation of microdosimetry results; of AlfaMC validation, a code designed with a focus on reducing the calculation time.