The Radiobiology of Particle Therapy

This chapter highlights the most relevant radiobiological characteristics of particle irradiation, trying to focus on the differences between low and high Linear Energy Transfer (LET). The kind of DNA‐damage induced by ionizing radiation plays an essential role in determining its effect. The radiation dose necessary to exert the same tumor cell killing effect is lower for high‐LET than for low‐LET radiation. In vitro data consistently confirm that high LET has greater efficacy in killing cells, that the effect is less dependent on hypoxia and viability of apoptotic pathways and that the mechanism of cell death differs from that of low‐LET irradiation. Relative biological effectiveness is the ratio between the physical absorbed dose of a reference radiation and of a test radiation that produce the same effect. The bystander effect is a biological effect on non‐irradiated cells induced by signaling from irradiated cells.