Microdosimetric analysis of (211)At in thyroid models for man, rat and mouse

BACKGROUND: The alpha particle emitter (211)At is proposed for therapy of metastatic tumour disease. (211)At is accumulated in the thyroid gland in a similar way as iodine. Dosimetric models of (211)At in the thyroid are needed for radiation protection assessments for 1) patients receiving (211)At-labelled pharmaceuticals where (211)At may be released in vivo and 2) personnel working with (211)At. Before clinical trials, preclinical studies are usually made in mice and rats. The aims of this study were to develop thyroid models for mouse, rat and man, and to compare microdosimetric properties between the models. METHODS: A thyroid follicle model was constructed: a single layer of 6 to 10-μm thick follicle cells with centrally positioned 4 to 8 μm (diameter) spherical nuclei surrounded a 10 to 500 μm (diameter) spherical follicle lumen. Species-specific models were defined for mouse, rat and man. The source compartments for (211)At were the follicle lumen, follicle cells and follicle cell nuclei. The target was the follicle cell nucleus. Simplified species-specific thyroid models were used to investigate the contribution from surrounding follicles. Monte Carlo simulations were performed using the general purpose radiation transport code MCNPX 2.6.0. RESULTS: When (211)At was homogeneously distributed within the follicle lumen, the mean specific energies per decay, 〈z〉, to the follicle cell nucleus were 2.0, 1.1 and 0.17 mGy for mouse, rat and man, respectively. Corresponding values for the single-hit mean specific energy per decay, 〈z(1)〉, were 1.3, 0.61 and 0.37 Gy. Assuming a homogeneous (211)At concentration in the follicle lumen, <0.5%, 7%, and 45% of the emitted alpha particles were fully stopped within the follicle lumen for the respective models. CONCLUSIONS: The results clearly show the influence of the follicle size, alpha particle range and (211)At location within the thyroid follicle on the dosimetric parameters. Appropriate thyroid models are required for translation of dosimetric parameters between species.