Abstract
In clinical use of heavy-ion beams, we cannot overlook skin damage appearing in the early time. We previously experimented effects of fractionated irradiation on the mouse skin with SOBP carbon-ion beams of 290 Mev / u . As an abrupt increase/decrease of normal tissue damage is caused by changing the number of fractions, we have concluded that, unlike photon therapy, skin damage should be carefully studied when the number of fractions is changed in new clinical trials. It is, however, still unclear whether any specific LETs cause such an abrupt change in skin reaction. Here we irradiated mice feet with mono-peak carbon ions, LET spectrum of which is much narrower than SOBP carbon ions, to obtain dose response of early skin reaction. Single doses and daily fractionation were used.
We got the following results. With an increase of LET, repair of skin damage caused by radiation decreased. Analyzing isoeffect doses, α/β ratios of LET13.6 kev/μm , 28.4 kev/μm and the gamma rays were not different between each others. However, irradiation with LET 58 kev/μm carbon beams produced much stronger effect on skin so that Fe-plot, which was successfully brought α/β ratios for lower LET carbon beams, could not straightforwardly produce α/β ratios. Skin RBE was least affected by fractionation for LET lower than 28.4 kev/μm. In case of LET 58 kev/μm, however, RBE value of ~3 for single doses increased to 4~4.5 for fractionated irradiation. This result may account for clinical carbon therapy of lung tumors that requires larger doses than doses predicted by extrapolating from fractionation date.
