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).