Cytogenetic damage after 131-iodine treatment for hyperthyroidism and thyroid cancer - A study using the micronucleus test

To detect the incidence and persistence of potential chromosome damage indu ced by iodine-131 therapy, we applied the cytokinesis-block micronucleus as say to peripheral blood lymphocytes from hyperthyroidism and thyroid cancer patients treated with I-131. Two groups of patients were evaluated in a lo ngitudinal study; one group was composed of 47 hyperthyroid patients and th e other of 39 thyroid cancer patients. In the hyperthyroidism group, the mi cronuclei frequency was determined before I-131 therapy and 1 week, 1 month and 3 months after it. Furthermore, an additional sample was taken from a subgroup of 17 hyperthyroidism patients 6 months after treatment. In the th yroid cancer group, the analysis was also conducted over time, and four sam ples were studied: before treatment and 1 week, 6 months and 1 year later. Simultaneously, a cross-sectional study was performed with 70 control subje cts and 54 thyroid cancer patients who had received the last therapeutic do se 1-6 years before the present study. In the hyperthyroidism group a signi ficant increase in the micronuclei average was found over time. In the samp le obtained 6 months after therapy, the micronuclei mean frequency was prac tically the same as in the sample taken 3 months before. In the thyroid can cer group a twofold increase in the frequency of micronuclei was seen 1 wee k after therapy. Although this value decreased across time, the micronuclei frequency obtained 1 year after I-131 therapy remained higher than the val ue found before it. Concerning the data from the cross-sectional study, a s ignificant increase in the frequency of micronuclei was detected in the sub group of thyroid cancer patients treated between 1 and 3 years before the c urrent study. These results indicate that exposure to I-131 therapy induces chromosome damage in peripheral lymphocytes and that the cytokinesis-block micronucleus assay is sensitive enough to detect the generic damage by exp osure to sufficiently high levels of radiation from internal radioactive so urces.