Planning of 131I therapy for graves disease based on the radiation dose to thyroid follicular cells.

Abstract

We evaluated the effects on the absorbed dose to thyroid follicular cells of self-absorption of (131)I radiation (specifically, beta-rays) in the follicular colloid. Thyroid follicles were modeled as colloid-filled spheres, containing a uniform concentration of (131)I and surrounded by a concentric monolayer of cells. Assuming close packing of identical follicles, we used Monte Carlo simulation to assess the absorbed dose to follicular cells. Because of beta-ray self-absorption in colloidal spheres with radii larger than 50 mum, the absorbed dose to follicular cells is less than the average thyroid absorbed dose. For the same thyroid mass, radioiodine thyroid uptake, and effective half-life, patients with follicles with colloidal sphere radii of 100, 200, 300, and 400 microm should be administered 9%, 15%, 21%, and 30% more (131)I, respectively, than patients with colloidal sphere radii of less than 50 microm, to yield the same absorbed dose to follicular cells.