engleski

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

In radiotherapy of Graves’ disease it is possible to assess the average dose to thyroid, which may differ from the dose to thyroid epithelial cells. Based on our clinical results that hypoechogenic thyroids are more radiosensitive than the normoechogenic ones and due to positive correlation of thyroid echogenicity and follicular size, we hypothesized that self-absorption of 131I radiation in colloid droplets diminishes the dose to epithelial cells. Methods: The follicles were modeled as spherical colloid droplets, containing uniform concentration of I-131, surrounded by monocellular layer of cells. We used the Monte Carlo simulation to assess the self-absorbed fraction in colloid and the dose to thyroid epithelial cells arising from the own colloid droplet (self-dose) and from the neighboring follicles (cross-dose). Results: Due to self-absorption in colloid droplets the dose to thyroid cells is less than the average thyroid dose, which becomes important when the radii of the droplets exceed 50 μ m. The patients having the average colloid droplets radii of 100, 200, 300 and 400 μ m should be given about 9, 15, 21 and 30% more activity of 131I per unit mass uptake and effective half-life than the patients with small colloid droplets, in order to produce the same dose to epithelial cells. Conclusion: The assessment of follicular size, possibly by ultrasound examination, should be incorporated in patient-specific 131I radiotherapy of patients with Graves’ disease.

Monte Carlo; diffuse toxic goiter; absorbed dose; echogenicity; thyroid follicle

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