Microdosimetric modeling of thyroid follicles for At-211

• At-211 is more frequently proposed for and used in targeted radionuclide tumour therapy, and is partly taken up by the thyroid gland in a similar way as iodine. For radiation protection purposes it is thus important to have appropriate dosimetric models of At-211 in the thyroid, both for dosimetric evaluation for patients receiving At-211 labelled pharmaceuticals, where the compound in vivo may release some At-211, and for the personnel handling At-211. The aims of this study were to describe a thyroid model for mice, rats and humans and to perform dosimetric calculations for At-211 to study the effect of geometry and biodistribution on absorbed dose/microdosimetric parameters.Material and MethodsSimulations were performed using the general purpose Monte Carlo code MCNPX 2.6.0. The decay of At-211 was simulated taking only the alpha particle emissions into consideration. A thyroid follicle was built up using spherical geometry. The follicle lumen was assumed to be a sphere with the diameter ranging between 10 and 500 micrometer, surrounded by a single layer of follicle cells with 4, 6 or 8 micrometer (diameter) spherical nuclei. Follicle models were created for mouse, rat and human, and for each geometric setup a number of 10.000.000 histories were simulated. The target was the follicle lumen and follicle cell nucleus and the sources of the At-211 decay were the follicle lumen, follicle cells and nuclei.ResultsComparing the human, rat and mouse model, with At-211 homogeneously distributed within the follicle lumen. The calculated mean specific energy for the follicle cell nucleus was 0.17, 1.1 and 2.0 mGy, respectively. The corresponding single-hit mean specific energy was 370, 614 and 1252 mGy, respectively. Approximately 45 % of the alpha particles in a At-211 decay for the human model deposit all the kinetic energy within the follicle lumen, compared to 7 % for the rat model and <0.5 % for the mouse model.ConclusionThe results demonstrate the importance of taking the range of the alpha particles, and the spatial distribution of At-211 into account for dosimetric evaluation of thyroid exposure. The dosimetry for At-211 in the thyroid for different species cannot readily be translated between the species.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Radiologi och bildbehandling (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Radiology, Nuclear Medicine and Medical Imaging (hsv//eng)

Nyckelord

Microdosimetry

At-211

Thyroid follicles

Publikations- och innehållstyp

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