RADIOTOXICITY AFTER I-131 THERAPY FOR THYROID-CANCER USING THE MICRONUCLEUS ASSAY

The purpose of the present study was to evaluate the degree of cytolog ical radiation damage to lymphocytes after I-131 therapy using the cyt okinesis-blocked micronucleus assay. The chromosomal damage to lymphoc ytes induced by I-131 in vivo should result in augmentation of the cel ls with micronuclei. Methods: We studied 25 patients with differentiat ed thyroid carcinoma who were treated with 3.7 GBq of I-131, Isolated lymphocytes collected from patients 1 wk after therapy were harvested and treated according to the cytokinesis-blocked method of Fenech and Morley, The micronucleus number of micronuclei per 500 binucleated cel ls were scored by visual inspection. As controls, lymphocytes from the same patients before therapy were also studied, In an in vitro study, lymphocytes from three patients at least 3 mo after therapy were expo sed to doses varying from 0.25 to 1 Gy and studied with the same metho d. Results: The mean number (mean +/- s.d.) of micronuclei after treat ment was significantly increased (p < 0.05) as compared to control sub jects (15.7 +/- 2.7 vs, 5.4 +/- 1.4), Since there was an interval rang ing from 6 to 20 mo (mean 11.8 mo) between the present and the last ra dioiodine therapy, no significant effect on the frequency of micronucl eus with cumulative radiation exposure of I-131 to lymphocytes was det ected, Internal radiation absorbed doses estimated for 25 patients wer e 0.33 +/- 0.09 Gy in this external irradiation study. Conclusion: The relatively low frequency of lymphocyte micronuclei induced by I-131 i n vivo and lack of significant effect on the frequency of lymphocyte m icronuclei with cumulative I-131 supported the contention that short-t erm nonstochastic damage of this therapy with 3.7 GBq of I-131 in thyr oid cancer patients is minimal and reversible.