Biological Gain of Carbon-ion Radiotherapy for the Early Response of Tumor Growth Delay and against Early Response of Skin Reaction in Mice

LET/Intra-track damage/Inter-track damage/Repair. The biological effectiveness of carbon ions relative to γ rays (RBE) was compared between the tumor growth delay and an early skin reaction of syngeneic mice. The RBE was larger for a tumor than skin when irradiated with large doses of high-LET (linear energy transfer) carbon ions. The intra-track damage ( α term of a linear quadratic model) of a tumor and skin increased equally with an increase of the LET, while the inter-track damage ( β term) of skin alone increased with the LET. These data provide evidence that high-LET radiotherapy could achieve therapeutic gain by minimizing the difference in response to fractionated irradiation between the tumor and normal tissue.