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 investigators. The aim of this study is to determine the frequency of chromosomal aberration in peripheral blood lymphocytes of patients treated by radioiodine (131I) for hyperthyroidism, with or without previous thyroid scan with 99mTc. Materials and Methods: Venous blood samples were obtained from 35 patients one month after radioiodine therapy and cytogenetically evaluated using analysis of metaphase in two groups. The first group (n = 15, 13 females and 2 males, mean age= 44.7 ±11.5 years and mean weight 74.4±7.9 Kg) received 5 mCi 99mTc for thyroid scanning 38.6±19.9 days before radioiodine therapy with 10.4 ± 3.4 mCi 131I. The second group (n = 20, 14 females and 6 males, 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 Females and 8 males, 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 significantly higher in patients who had not received 99mTc compared those who had undertaken thyroid scan before radioiodine therapy (p=0.03). Conclusion: These findings may indicate the fact that the radiation dose received from 99mTc could induce resistance to subsequent higher radiation dose of 131I in peripheral blood lymphocytes and it might be due to cytogenetic radioadaptive response. Iran . J. Radiat. Res., 2004 2 (2): 69-7 4