Does Initial or Persistent DNA damage Correlate With the Enhanced RBE of Carbon Versus Photon Radiation Therapy?

Abstract

Purpose/Objective(s): Induction of DNA-double-strand-breaks (DSB) is considered a determinant of radiation-mediated cell killing. The relative biological efficacy (RBE) of high linear energy transfer (LET) irradiation versus low-LET irradiation is estimated to be w3. However, quantitative analysis of DSB directly after irradiation could not explain the biological differences in clonogenic survival. The higher complexity of DSB, reported as prolonged or persistent DNA damage after high-LET irradiation, is a candidate determinant of its enhanced RBE. We therefore sought to validate the role of complex DNA damage and differences in repair kinetics in radiation cell killing by correlating clonogenic survival with both initial and persistent radiation-induced DSB. Materials/Methods: To this end, human non small cell lung cancer cells (A549), epidermoid cancer cells (A431) and human umbilical vein endothelial cells (HUVEC) were irradiated with either high-LET Carbon ions (260 MeV/u, 100 keV/mm) or low-LET photon (250 keV X-rays, 2 keV/mm) irradiation. Sma-560 mouse glioma cells were irradiated with carbon irradiation of different LET (30 170 keV/mm). DSB were detected by gamma-H2AX staining followed by microscopy and flow cytometry at different time points post-irradiation. The persistent damage fraction (PDF) was determined as the ratio of DSB detected at 24 hours versus 30 minutes post-irradiation. To detect biological differences in radiation response, clonogenic survival assays were performed in parallel. Results: DNA repair kinetics analysis confirmed the high amount of persistent damage for high-LET carbon irradiation compared to low-LET irradiation. The RBE for clonogenic survival was between 2.0-2.5 for highLET carbon irradiation among different cell types. The “Damage RBE” ranged from 1.8 2.2 for initial damage and from 2.5 5.1 for persistent damage. In contrast, in Sma-560 and for carbon irradiation with a range of different LET, the RBE for clonogenic survival correlated with persistent DNA damage. The PDF showed a direct dose-dependency for low-LET irradiation (range from 5 20%), while the PDF for high-LET carbon irradiation was independent of dose at w20%. Conclusions: Quantitative DSB-repair studies with different LET carbon irradiation, so far performed only in Sma-560, indicated that the amount of persistent DNA damage, but not initial DNA damage may correlate with radiation cell killing of high-LET carbon irradiation. The PDF was dosedependent and low for low-LET irradiation while dose-independent and high for high-LET irradiation. These data suggest that neither initialnor persistent DNA damage may provide a plausible explanation for the enhanced RBE of high-LET carbon versus low-LET photon irradiation. Author Disclosure: P. Seidel: None. J. Debus: None. A. Abdollahi: None.