Adaptive response in patients treated with I-131

The aim of this study was to investigate whether an adaptive response (defi ned as the induction of radiation tolerance after a small dose of radiation ) could be observed in peripheral blood lymphocytes of patients treated wit h I-131 for thyroid disease. Methods: For each patient, blood samples were taken immediately before and 1 wk after I-131 administration. Each blood sa mple was divided into 3 fractions and the fractions were subsequently irrad iated in vitro with 0, 0.5, and 1.0 Gy Co-60 gamma-rays. After blood cultur e for 70 h, cells were harvested and stained with Romanowsky-Giemsa and mic ronuclei were counted in 1000 binucleated cells. The increase in micronucle i by the in vitro irradiation of the blood samples taken before and after t herapy was compared. In this setup, an adaptive response is represented by a significant decrease of the in vitro induced micronucleus yield after the rapy compared with that before therapy. The iodine therapy can be considere d as an in vivo adaptation dose, after which the subsequent in vitro irradi ation acts as a challenge dose. To investigate the reproducibility of the m ethod, 2 subsequent blood samples of healthy volunteers were taken 7 d apar t. Irradiation and cell culture were performed as described. Results: In 8 of 20 patients, a significant (P = 0.0002) decrease was found in the in vit ro induced micronucleus yield in the blood sample taken 1 wk after I-131 ad ministration compared with that of the blood sample taken before therapy. N o significant (P > 0.1) differences were observed between these 8 patients and the other patients when the number of micronuclei induced in vivo by th e iodine treatment and the resulting equivalent total body dose were compar ed. None of the control subjects showed a significant change in micronucleu s yield after in vitro irradiation between both blood samples taken 1 wk ap art. Conclusion: The iodine treatment can act as an in vivo adaptation dose and can induce an adaptive response that is observed by a decrease of the cytogenetic damage in peripheral blood lymphocytes after in vitro irradiati on as challenge dose. A large interindividual difference was observed.