In radiobiology, the α-β ratio, defined as the dose at which killing of cells by linear (α) and quadratic (β) components are equal, is used to quantify the fractionation sensitivity of tissues and tumours. With data from low-dose-rate brachytherapy and external-beam radiotherapy series, Brenner and Hall were the first to report a low α-β ratio of 1•5 for prostate cancer; many investigators have since recorded similar values (lower than those of surrounding normal tissues, such as the rectum and bladder)....