Cell survival curves derived from early and late skin reactions in patients

The survival curve of the epidermal stem cells in patients was derived from the study of skin reactions after fractionated radiotherapy. This approach implies the basic hypothesis that the same level of skin reaction corresponds to the same level of survival of the stem cell population. Skin reactions in patients were compared on symmetrical areas irradiated with fractionation schemes consisting of N and 2N fractions delivered over the same period in order to eliminate the influence of cell proliferation. From the comparison, the isoeffect dose for early skin tolerance (dry/moist desquamation) could be obtained as a function of fraction number or fraction size. It was then possible to derive the ratio between the final and initial slope of the stem-cell survival curve (1D0/D0), which was found to be equal to 3. For late skin reactions, the influence of the dose per fraction was stronger than for the early reactions, reflecting a smaller initial slope (larger 1D0) to the cell survival curve. (author)