Can previous thyroid scan induce cytogenetic radioadaptive response in patients treated by radioiodine for hyperthyroidism?

Background:Induction of radioadaptive responses in cells pretreated with a low dose radiation before exposure to a high dose is well documented by many investigations. The aim of this study is to determine the frequency of chromosomal aberration in peripheral blood lymphocytes of patients treated by radioiodine I for hyperthyroidism, with or without previous thyroid scan with Tc. Material and Methods:Venous blood samples were prepared from 35 patients one month after radioiodine therapy and cytogenetically evaluated using analysis of metaphase in two groups.The first group ( N = 15, 13 female and 2 male, mean age = 44.7 ±11.5 years and mean weight 74.4±7.9 Kg) received 5 mCi Tc for thyroid scanning 38.6±19.9 days before radioiodine therapy with 10.4 ± 3.4 mCi I . The second one (N = 20, 14 female and 6 male, mean age = 41.0 ± 10.8 years and mean weight = 68.1±9.2 Kg) didn't have history of thyroid scanning. We also studied a control group (N = 29, 11 Female and 8 male, mean age = 33.7±7.4 and mean weight = 70.0±8.8 Kg) who didn't have any history of diagnostic or therapeutic and also occupational exposure. Results:The mean frequency of total chromosomal aberrations in the first and second groups and controls were 1.46 ±1.55, 1.65 ± 1.62 and 0.93 ± 0.92 respectively. Results also showed that the mean frequency of total chromosome aberration in two groups were higher than controls and also it was statistically significant higher in patients who had not received Tc compared with the ones who undertaken thyroid scan before radioiodine therapy (p=0.03). Conclusion:The above findings may indicate the fact that the radiation dose received from Tc could induce resistance to subsequent higher radiation dose of I in peripheral blood lymphocytes and it might be due to cytogenetic radio-adaptive response.