Retrospective Biological Dosimetry at Low and High Doses of Radiation and Radioiodine Impact on Individual Susceptibility to Ionizing Radiation.

Iodine-131 (I-131) is often used in thyroid diagnostics and therapy. External and internal exposure to radioiodine can lead to molecular and cellular damage in peripheral blood lymphocytes. The aim of this study was to explore the influence of low and high doses of I-131 on susceptibility to ionizing radiation. Study groups consisted of 30 individuals free of thyroid diseases, 41 patients exposed diagnostically to low doses of I-131, and 37 hyperthyroidism patients exposed therapeutically to high doses. The standardized DNA repair competence assay was used to test the efficacy of the fast DNA repair process in G₀ cells. Cytogenetic preparations were made in fresh blood samples before and after challenging cells in vitro with X-ray dose. The frequency of sister chromatid exchanges (SCE) and percentage of cells with significantly elevated numbers of SCE were used as cytogenetic biomarkers associated to homologous recombination and compared to reported earlier cytogenetic biomarkers of cancer risk. Strong individual variation in the biomarkers is observed in all investigated groups before and after challenging. Nevertheless, the efficiency of post challenging fast repair is significantly high in the patients exposed to diagnostic I-131 doses than in unexposed control group and linked to decreased cytogenetic damage. However, 5 weeks after administration of therapeutic doses, significant increases of unrepaired post challenging DNA and cytogenetic damages were observed indicating a health risk. Results also suggest that the appearance of cancers in immediate families might influence DNA repair differently in patients exposed to low than to high doses.