Comparison of clonogenic cell survival and DNA damage induced by ¹⁸⁸Re and X-rays in rat thyroid cells

Ionizing radiation produces DNA lesions among which DNA double strand breaks (DSB) are the most critical events. Radiation of various energy types might differ in their biological effectiveness. Here, we compared cell survival and DNA damage induced by ¹⁸⁸Re and X-rays using γH2AX foci as a measure of DSB. The correlation between survival and residual foci was also analyzed.

PCCl3 cells were irradiated with 200 kV X-rays (1.2 Gy/min) or 0.5-25 MBq/ml ¹⁸⁸Re (1 h irradiation) achieving doses up to 10 Gy. By blocking of sodium iodide symporter (NIS) essentially extracellular activity could be guaranteed. Survival fractions (SF) were detected by colony forming assay. Initial and residual γH2AX foci (15 min and 24 h after irradiation) were assessed by immunostaining. The relationship between SF and residual radiation induced γH2AX foci (RIF) was evaluated by Spearman and Pearson correlation tests.

We did not find significant differences between the survival curves in terms of the radiation quality. The D₃₇ values were 4.6 Gy and 4.2 Gy for ¹⁸⁸Re or X-ray, respectively. The initial foci numbers were in the same range for ¹⁸⁸Re and X-ray, but higher levels of residual foci persisted after X-rays in comparison to ¹⁸⁸Re (1 Gy_X-ray 6.5 ± 0.2; 1 Gy_Re-188 4.8 ± 0.2 RIF). Accordingly, for ¹⁸⁸Re a higher extent of DSB repair was found. The Spearman test revealed a significant (p < 0.01) correlation between SF and residual RIF for both radiation modalities.

No differences in terms of radiation were found for SF and initial foci. However, residual foci were lower for ¹⁸⁸Re than for X-rays. A prediction of SF by residual foci should consider the properties of the radiation qualities that influence foci removal and DSB repair.