Agreement of quadratic and CRE models in predicting the late effects of continuous low dose-rate radiotherapy

It has been known for some time, and is now generally accepted, that normal tissue isoeffect formulae based on the Ellis equation (Ellis, 1969) including the CRE and TDF formulations do not correctly account for the late-occurring effects of fractionated radiotherapy (Bates & Peters, 1975; Singh, 1978; Turesson, 1978). Correction factors have been proposed (Turesson & Notter, 1979) to take account of the increased effect on late-responding tissues of large fractions but this approach is both unwieldy and unsatisfactory from a philosophical point of view. Alternatively, the exponents in these power function models may be varied depending on the tissue in question (Wara et al, 1973; Pezner & Archambeau 1981).