Objectives: Radiation-induced DNA double strand breaks (DSBs) can be immunofluorescently stained as microscopic foci using γ-H2AX and 53BP1 antibodies, which allows quantification of radiation effects. We investigated DSB formation by internal irradiation of blood lymphocytes with ⁶⁸Ga to generate a DSB focus calibration-curve for a β⁺-emitter in-vitro.

Methods: Five blood samples were taken from four healthy volunteers at different days. ⁶⁸Ga of known activity was mixed to the blood samples to achieve absorbed doses up to 100 mGy after 1h incubation. For the individual background focus rate one non-irradiated sample of each volunteer was kept. The lymphocytes were separated by density centrifugation, washed in PBS and fixed in ethanol. After two-color immunofluorescent staining the co-localizing γ-H2AX and 53BP1 foci were counted manually using a red/green double band pass filter. To calculate the absorbed doses, the activity in each blood sample was determined with a calibrated germanium detector.

Results: 31 blood samples (absorbed doses up to 108 mGy) were evaluated. In agreement with our established in-vitro calibration-curve for the β^--emitters I-131 and Lu-177, a linear regression fitted to the data showed a good correlation between the number of RIF/cell and the absorbed doses to the blood. However, the slope of the ⁶⁸Ga calibration-curve (0.011 RIF/cell·mGy^-1) was 23% less compared to the value obtained for the β^--emitters. Since the mean positron energy emitted by ⁶⁸Ga (829keV) is higher compared to the mean electron energy of I-131 (182keV) and Lu-177 (134keV) we expect a different energy deposition pattern which may lead to an altered DSB foci yield.

Conclusion: Our results show that there is a decrease of the number RIF with increasing energy of the β^-/β⁺-emitters. Further experiments with nuclides of varying particle emission energies are needed to verify whether there is an energy dependent DNA damage response after internal irradiation.