DOSIMETRY AND RISK ESTIMATES OF RADIOIODINE THERAPY FOR LARGE, MULTINODULAR GOITERS

In patients with a large, multinodular goiter (>100 g), radiation abso rbed doses in the thyroid, surrounding tissues and remainder of the bo dy were estimated after therapeutic administration of I-131(3.7 MBq or 100 mu Ci/g of thyroid tissue retained at 24 hr), Methods: Thermolumi nescent dosimeter (TLD) measurements were performed on 23 patients (12 euthyroid and 11 hyperthyroid; thyroid weight 222 +/- 72 g; 2.1 +/- 0 .9 GBq I-131) on the skin over the thyroid, over the submandibular gla nd 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 I-131). An iodine biokinetic model and the MIRD methodology were used to estimate absorbed doses in organs. C ancer risks were calculated using ICRP Publication 60, Results: Cumula ted 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 ra dioiodine retained in the thyroid at 24 hr (euthyroid versus hyperthyr oid not significant), Absorbed doses in the thyroid of 94 +/- 25 Gy fo r 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-lime risk of development of cancer outside the thyroid g land was calculated, When applied to people of 65 yr and older the est imated risk is approximately 0.5%, Conclusion: These data may help in choosing the treatment regimen for individual patients with a large, m ultinodular goiter, who have to be treated for hyperthyroidism or comp ressive problems. In younger patients, surgery may be preferred. Howev er, for elderly patients and patients with cardiopulmonary disease, th e advantages of noninvasive radioiodine treatment will outweight the l ife-time risk of this mode of therapy.