Low dose per fraction in radiotherapy: a definition for 'flexure dose'

The use of 0.1 α/β is proposed as a practical definition of flexure dose, where α and β are the coefficients of the linear and quadratic terms in the relevant dose-response equation. α/β has the dimensions of dose, with values of about 600-1200 cGy for early-reacting tissues and of 100-500 cGy for late-reacting tissues. The choice of the factor 0.1 implies that a difference of 10% in total iso-effect dose can be detected with acceptable significance in two schedules with different fraction numbers. α and β are defined for the functional dose-response curve of a tissue, not necessarily for the clonogenic cell survival curve. (U.K.)