This document discusses several key concepts in radiobiology including: 1. The interaction of radiation with cells is probabilistic, with damage occurring through direct and indirect action. Indirect action involves free radicals produced by radiation interacting with water molecules within cells. 2. Different phases of the cell cycle have differing radiosensitivities, with G2/M being most sensitive. Fractionated radiation can exploit this through redistribution effects. 3. The linear quadratic model describes cell survival curves and accounts for both single-hit and double-hit damage from radiation. It is used to calculate biologically equivalent doses. 4. Mechanisms like reoxygenation between fractions can improve the therapeutic ratio by making tumor cells