Low sensitivity of the sister chromatid exchange assay to detect the genotoxic effects of radioiodine therapy

To assess the genotoxic risk associated with I-131 therapy, sister chromati d exchanges (SCEs) and cells with unusually high SCE counts (HFC) were dete rmined in a follow-up study performed with 46 hyperthyroidism and 39 thyroi d cancer patients treated with I-131. In addition, a cross-sectional study was also carried out with 78 control persons and 51 thyroid cancer patients that had completed radioiodine therapy from 1 to 6 years prior to the curr ent investigation. In the follow-up analysis, the study was conducted over time and four blood samples were drawn from each patient: the first one pri or to the radioiodine treatment, with the remaining three taken sequentiall y over the year after therapy. Concerning the results obtained for the whol e population in the follow-up study, the SCE and HFC values found after rad ioiodine therapy did not show any significant increase, neither in the hype rthyroidism nor thyroid cancer groups. Unlike the results mentioned above, when the effect of smoking habit was considered, there was a slight but sig nificant increase in SCE in the samples taken 3 months and 1 week after I-1 31 therapy in the hyperthyroidism and thyroid cancer non-smokers, respectiv ely. The data obtained in the cross-sectional study did not show difference s in SCE and HFC between the control group and the cancer group treated wit h I-131. It is noteworthy that among the different parameters analysed, smo king habit is the only factor that showed a direct relationship with SCE an d HFC and, as a consequence, smokers had significantly more SCE and HFC tha n nonsmokers. Taking into account our previous investigations showing a hig hly significant increase in the frequency of micronuclei for the same patie nts and sampling times, the outcomes obtained would suggest that the eventu al genotoxic effect of I-131 therapy could not be clearly detected by the S CE assay, This would reinforce the view that ionizing radiation appears to be a poor inducer of SCEs.