Human skin: early and late reactions in relation to dose and its time distribution

Various reactions of the skin are observed after irradiation. Erythema and pigmentation are transient effects without any lasting consequence. Desquamation is a consequence of the cell kill produced in the basal layer of the epidermis; the mechanism and time-course of the reaction are fairly well understood. Late reactions appear 3 months after the acute reaction. The risk of necrosis reaches a peak at 3 years. Damage to the dermis is the main cause of the late reactions. The mechanism is complex, involving various tissue substructures, and has not yet been well analysed. The time distribution of the dose is of paramount importance in the reactions. Numerous experimental and clinical studies have provided data on the variation of tolerance dose with time factors. They have demonstrated the difference of response to time factors for early and late reactions. Fractionation and dose rate (involving cell repair) have a larger effect on late reactions, while overall time (involving cell multiplication) has a larger effect on early reactions. This finding has brought some clarity to the conflicting conclusions of clinical studies comparing various schedules. (author)