Abstract
Abstract
Purpose: To clarify the properties of high-energy protons in in situ DNA double strand breaks (DSBs), apoptosis induction, as well as clonogenicity with a comparison to high-energy X-rays.
Materials and methods: Two human tumor cell lines ONS76 and MOLT4 were irradiated with10 MV X-rays or 200 MeV proton beams. In situ DNA double strand breaks were evaluated by immunocytochemical analysis of γ-H2AX foci in both cell lines. Yield of apoptosis was measured by flowcytometry after Annexin-V and PI staining using MOLT4. Standard clonogenic survival assay was performed to obtain relative biological effectiveness of protons to X-rays using ONS76.
Results: Yield of γ-H2AX foci were always higher in proton beam irradiation than in X-ray in both cell lines with factors being 1.14-1.436. γ-H2AX foci formation reached their peak at 30 minutes after irradiation in both cell lines and after this point, they decreased gradually. Proton beams induce apoptosis earlier than X-rays and the final yield of cell death was significantly greater in protons compared to X-ray irradiation. The ratios of apoptosis induced by proton beams and X-rays were 1.01-1.52 at 12 hour after irradiation in MOLT4. RBE of 200MeV proton to 10 MV X-ray at 10% survival and ratio of SF2 in clonogenic survival assay were 1.067 and 1.111, respectively.
Conclusions: Although the proton RBE values in the colony formation assay was within the range previously reported, the induction of DNA double strand breaks and yield of apoptosis were significantly higher in proton irradiation than in X-ray irradiation. Thereby, the underlying mechanisms in cell inactivation may differ between these radiation qualities.
