Planning of 131I Therapy for Graves Disease Based on the Radiation Dose to Thyroid Follicular Cells: TABLE 1

We evaluated the effects on the absorbed dose to thyroid follicular cells of self-absorption of 131I radiation (specifically, β-rays) in the follicular colloid. Methods: Thyroid follicles were modeled as colloid-filled spheres, containing a uniform concentration of 131I 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. Results: Because of β-ray self-absorption in colloidal spheres with radii larger than 50 μm, the absorbed dose to follicular cells is less than the average thyroid absorbed dose. Conclusion: 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 μm should be administered 9%, 15%, 21%, and 30% more 131I, respectively, than patients with colloidal sphere radii of less than 50 μm, to yield the same absorbed dose to follicular cells.