Radiation physics and radiobiology.

Three quite general rules link radiation physics to radiobiology. They concern the dependence on linear energy transfer of relative biological effectiveness and of the cross section for cell killing, as well as the dependence of relative biological effectiveness on absorbed dose. These rules are accounted for in compound dual radiation action according to which damage in the nanometer domain depends linearly on dose with no dose rate dependence and on relative biological effectiveness that is limited to low values because of saturation. Energy concentration in the micrometer domain can cause large relative biological effectiveness in processes in which pairs of DNA lesions interact with quadratic dose dependence and dose rate dependence for low linear energy transfer radiations. Damage at both the nanometer and the micrometer level can cause observed effects and their relative contributions determine the maximum relative biological effectiveness at very low doses.