Effects of radiation on tissues are generally mediated by one of two mechanisms:
♦ Loss of mature functional cells by apoptosis (active form of cell death, usually within 24 hours of irradiation).
♦ Loss of reproductive capacity.
Different cell types show large differences in radiosensitivity to either of these processes and only a limited number of cell types predominantly respond by apoptosis. These include some cells of haemo-poietic lineage and salivary glands. As most tissues or organs have redundant functional cells, they may lose a significant fraction of this cell population by apoptosis without clinical impairment of tissue function. Usually lost cells are replaced by proliferation of surviving stem cells or progenitor cells. These may be cells surviving in irradiated tissue or cells migrating from unirradiated margins.
When cell loss occurs predominantly through loss of proliferative capacity, the rate of cell renewal (proliferation) of a particular organ determines the time of appearance of tissue damage, varying from days to even years after irradiation. This has led to the arbitrary distinction of acute and late effects of radiation, with acute effects being restricted to changes developing during a fractionated course of radiotherapy of 6-8 weeks.
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