Response of Tissues to Multiple Small Dose Fractions

Conventional radiotherapy uses multiple fractional doses each of which is small enough that cell killing occurs predominantly through events. If cellular survival from each fraction were plotted on a single dose-survival curve, it would be within the shoulder region. The initial slope, and the dose range over which survival begins decreasing more rapidly due to lethality from accumulated sublethal events, are the dose-survival parameters most relevant to radiotherapy. Several methods are used to quantify the response of tissues to doses in the range used clinically. At present, such dose-response parameters are subject to considerable uncertainty, but at least they provide a basis for improved understanding of tissue responses to radiation and a frame-work for further experimentation and, ultimately, for selecting optimum-sized dose fractions.