Radiotoxicity after iodine-131 therapy for thyroid cancer using the micronucleus assay

The purpose of the present study was to evaluate the degree of cytological radiation damage to lymphocytes after 131I therapy using the cytokinesis-blocked micronucleus assay. The chromosomal damage to lymphocytes induced by 131I in vivo should result in augmentation of the cells with micronuclei.

We studied 25 patients with differentiated thyroid carcinoma who were treated with 3.7 GBq of 131I. 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 cells 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 exposed to doses varying from 0.25 to 1 Gy and studied with the same method.

The mean number (mean +/- s.d.) of micronuclei after treatment was significantly increased (p < 0.05) as compared to control subjects (15.7 +/- 2.7 vs. 5.4 +/- 1.4). Since there was an interval ranging from 6 to 20 mo (mean 11.8 mo) between the present and the last radioiodine therapy, no significant effect on the frequency of micronucleus with cumulative radiation exposure of 131I to lymphocytes was detected. Internal radiation absorbed doses estimated for 25 patients were 0.33 +/- 0.09 Gy in this external irradiation study.

The relatively low frequency of lymphocyte micronuclei induced by 131I in vivo and lack of significant effect on the frequency of lymphocyte micronuclei with cumulative 131I supported the contention that short-term nonstochastic damage of this therapy with 3.7 GBq of 131I in thyroid cancer patients is minimal and reversible.