Abstract
CHO-K1 cells and HeLa cells were irradiated with low energy iron ions (0.93 MeV u-1) at the HIMAC in the NIRS. Cells were immunostained against γ-H2AX, as an indicator of DNA double-strand breaks (dsbs), to clarify the correlation between the expected average number of ion traversals in the nucleus and the observed number of γ-H2AX fluorescent spots. The correlation between the number of γ-H2AX spots observed and calculated ion traversals was approximately 1, indicating that dsbs were produced at each ion traversals. The cell inactivation cross-section (σ) obtained from the survival curves was 98.0 γm2, which was 0.45 of the average area of nucleus of HeLa cells, indicating that one lethal required 2.2 ion traversals. Further investigation with lighter ions, such as C, N, O, Ne, and Ar ion were also performed and the σ showed no less than half the size of nucleus area. Pulse field gel electrophoresis was also performed to measure the dsbs induction per ion track by FAR assay, and was roughly estimated be more than 20 dsbs per track. The amount of fragmented DNA did not decrease even with 6 hr incubation in culturing condition after irradiation. From these experimental results, three possibilities can be considered. One is, DNA may not be the sufficient factor for cell killing, and secondary lethal factor, yet not clarified, exist. Secondly, radiation sensitive site in the cell nucleus are approximately half the size of nucleus. Thirdly, contrary to the production of dsbs along the particle traversals, the area of the cell nucleus are not equally sensitive to radiation, and perhaps some kind of radiation sensitive distribution exists in the cell nucleus.
