Dosimetry and risk estimates of radioiodine therapy for large, multinodular goiters.

UNLABELLED In patients with a large, multinodular goiter (> 100 g), radiation absorbed doses in the thyroid, surrounding tissues and remainder of the body were estimated after therapeutic administration of 131I(3.7 MBq or 100 microCi/g of thyroid tissue retained at 24 hr). METHODS Thermoluminescent dosimeter (TLD) measurements were performed on 23 patients (12 euthyroid and 1I hyperthyroid; thyroid weight 222 +/- 72 g; 2.1 +/- 0.9 GBq 131I) on the skin over the thyroid, over the submandibular gland and over the parotid gland. Thyroid radioactivity measurements were done daily in 6 euthyroid and 6 hyperthyroid patients (thyroid weight 204 +/- 69 g; 1.9 +/- 0.9 GBq 131I). An iodine biokinetic model and the MIRD methodology were used to estimate absorbed doses in organs. Cancer risks were calculated using ICRP Publication 60. RESULTS Cumulated absorbed doses on the skin (TLD measurements) were 4.2 +/- 1.4 Gy (thyroid), 1.2 +/- 0.6 Gy (submandibular) and 0.4 +/- 0.2 Gy (parotid). All these values were significantly correlated with the amount of radioiodine retained in the thyroid at 24 hr (euthyroid versus hyperthyroid not significant). Absorbed doses in the thyroid of 94 +/- 25 Gy for euthyroid and 93 +/- 17 Gy for hyperthyroid patients were calculated (thyroid radioactivity measurements). Extrathyroidal absorbed doses (means of 12 patients) were 0.88 Gy in the urinary bladder, 0.57 Gy in the small intestine, 0.38 Gy in the stomach, and ranged from 0.05 to 0.30 Gy in other organs (euthyroid versus hyperthyroid not significant). A 1.6% life-time risk of development of cancer outside the thyroid gland was calculated. When applied to people of 65 yr and older the estimated risk is approximately 0.5%. CONCLUSION These data may help in choosing the treatment regimen for individual patients with a large, multinodular goiter, who have to be treated for hyperthyroidism or compressive problems. In younger patients, surgery may be preferred. However, for elderly patients and patients with cardiopulmonary disease, the advantages of noninvasive radioiodine treatment will outweight the life-time risk of this mode of therapy.