“…Radiation-induced apoptosis is found in hierarchical high-turnover adult tissues that are enriched for immature stem and progenitor cells such as bone marrow, spleen, thymus, and intestine, but not in low-turnover tissues such as heart, kidney, liver, skeletal muscle, and brain [7,8]. However, the dose required to kill a radiosensitive cell varies from tissue to tissue (relative radiosensitivity), and, as exemplified by radiation-induced apoptosis in the intestine, which is confined to a small subpopulation of cells in the crypt, and with the exception of cells in the spleen and bone marrow, which are all radiosensitive, the proportion of radiosensitive cells within a high-turnover tissue also varies [4,5,8].…”