Translocation frequencies measured in patients one year after radioactive iodine therapy for thyrotoxicosis

Purpose: To investigate the incidence of translocations induced by iodine-1 31 therapy in thyrotoxicosis patients 1 year after the administration of th e radiolabelled compound. Materials and methods: Tricolour FISH with whole-chromosome-specific probes for chromosomes 8 was used for scoring translocations. From the genomic tr anslocation frequencies, derived using the Lucas formula, equivalent whole- body doses were calculated, based on the in vitro Co-60 gamma -ray dose-res ponse curve. Results: A total of 101 translocations were observed in 4864 metaphases, 63 % being of the two-way type. In the control group used for obtaining dose-r esponse data, nine translocations were observed in 5278 metaphases, 55% bei ng two-way translocations. No correlation was found between the observed fr equency of translocations and administered radioactivity. Using the in vitr o dose-response, an estimated average dose for the group of nine patients o f 0.79 +/- 0.22 Gy was obtained. Compared with frequencies following the as sumption that the involvement of a particular chromosome in a two-break exc hange-type aberration is proportional to its DNA content, chromosome 4 was more frequently involved and chromosomes 2 and 8 less frequently involved i n chromosomal rearrangements. Conclusion: This study shows that I-131 therapy for thyrotoxicosis patients induced translocations, especially in chromosome 4, which could be detecte d 1 year after the administration of the radiolabelled compound.