Dose-Effect Relationships for Radiation Damage to Organized Tissues

The relation between dose of ionizing radiation and the survival of mammalian cells is well established. The survival curve has been observed for cells irradiated and cultured in vitro and in vivo. Although some authors assume that the extrapolation number, N, is usually near two for mammalian cells, observations made at the cellular level have demonstrated that higher values of N are frequently observed. The actual value of N for the cells in a given tissue is of great importance, both for theoretical reasons and for the practice of radiotherapy, and this article summarizes evidence from which it may be deduced thaf the relevant extrapolation number N may often be much greater than two. (H.M.G.)