Hengstler, J. G., Bockisch, A., Fuchs, J., Grimm, W., Görges, R., Oesch-Bartlomowicz, B., Zapf, A-O., Lade, K., Tanner, B., Teichmann, E., Thelen, M., Gebhard, S. and Oesch, F. Induction of DNA Single-Strand Breaks by 131I and 99mTc in Human Mononuclear Blood Cells In Vitro and Extrapolation to the In Vivo Situation. The radionuclides 131I and 99mTc are frequently used for therapy of benign and malignant thyroid disease (131I) and for diagnosis of thyroid and other diseases (99mTc). However, the levels of DNA single-strand breaks (SSBs) induced in cells of patients after administration of 131I and 99mTc are not known. In this study, we measured the number of SSBs per cell induced by 131I and 99mTc in vitro, extrapolated the results to the clinical situation, and assessed their biological relevance by comparing levels of SSBs induced after therapeutic administration of 131I and 99mTc to those induced by endogenous processes or by occupational exposure to genotoxic substances. A linear dose–response relationship between the radioactivity concentrations of 131I and 99mTc and SSBs in human mononuclear blood cells (determined by alkaline elution) was obtained after incubation at 4 and 37°C. At 4°C, where almost no repair of SSBs takes place, 131I and 99mTc induced 81 and 7 SSBs per cell per hour/(MBq/ml), respectively. At 37°C, only 20 and 1.6 SSBs per cell per hour/(MBq/ml) were observed after incubation with 131I and 99mTc. To estimate the induction of SSBs in vivo in cells of patients after administration of 3700 MBq 131I (oral) or 60 MBq 99mTc (i.v.), the rates of induction of SSBs obtained in vitro were extrapolated to the concentrations of 131I and 99mTc measured in blood of patients. The total number of SSBs (mean ± standard deviation) accumulated after oral administration of 3700 MBq 131I up to 70 h after administration was calculated as 200 ± 59 SSBs/cell. After administration of 60 MBq 99mTc (i.v.), 0.032 ± 0.009 SSBs per cell (total SSBs up to 2 h after administration) were cumulated. The induction of SSBs by endogenous processes (estimated 2,000 SSBs per cell per hour) and by occupational exposure to genotoxic substances (125–430 SSBs per cell) has been estimated in earlier studies. In conclusion, the frequency of SSBs induced by thyroid diagnosis with 60 MBq 99mTc is approximately 5 orders of magnitude smaller than the frequency of spontaneous SSBs and thus is most probably without biological relevance. Since the frequency of induction of SSBs by therapy with 131I (3700 MBq) is about 6000-fold higher compared to thyroid diagnosis by 99mTc, its biological relevance is more difficult to assess. Nevertheless, the number of SSBs induced by therapy with 131I is substantially lower than that induced by endogenous processes.