Abstract
When ion beams are irradiated to a cell layer placed parallel to the beam axis and γ-H2AX is visualized after the irradiation, one can speculate the depth-dependent localization pattern of DNA double strand breaks (DSBs) along the tracks of the beams. We examined the distribution pattern of γ-H2AX foci along the tracks of 200 MeV proton beams around the depth proximal to their range end.
Proton beams were irradiated to a layer of confluent BALB-3T3 cells through a solid water, thickness of which was adjusted to stop most of the projectiles with in the cell layer.
In order to clarify the morphology of γ-H2AX foci by single proton, proton beams in low fluence (about 10 protons / 100 μm2) was irradiated to the cell layer. We found that solid or dotted lined foci were formed in nuclei at the depth proximal to the peak of the absorption dose. We assumed those lined foci as a result of DSBs evoked along the track of single proton in the proximity of its range end. Length of the solid lined foci was not extend beyond 5 μm, corresponding to the high LET part ( > 50 keV/μm) of the proton beams in the proximity of their range end.
When the number of lined foci (solid and dotted lined foci) per an unit area was counted at various depth, it steeply rose just anterior to the depth of peak of the absorption dose, and reached maximum at the depth of the peak (or slightly posterior to the peak). Elevated number of the lined foci was sustained for several millimeter, while the absorption dose abruptly decreased.
We also report changes in the number of the foci, in total volume of foci, and the number and volume of long-lasting foci at the different depth around the peak of the absorption dose.
