THE DOSE-RATE EFFECT REVISITED - RADIOBIOLOGICAL CONSIDERATIONS OF IMPORTANCE IN RADIOTHERAPY

A wide range of dose-rates have been used in radiation biology and radiation therapy, extending from a few cGy per day to hundreds of Gy in a fraction of a second. The dose-rate range of importance in radiotherapy extends from about 0.1 Gy/hr to several Gy/min. In this range, the fraction of cells killed by a given dose decreases as the dose-rate is reduced, principally because of the repair of sub-lethal damage. In some cell lines, an inverse dose-rate effect is observed where, over a narrow range of dose-rates, the effectiveness of a given dose increases with decreasing dose-rate if cells move through the cycle and are arrested in G2, which is a radiosensitive phase. In recent years data have accumulated for cells of human origin. About 40 data sets have been analyzed for values of the survival curve parameters and the rate of repair of sub-lethal damage. These data have been used to address three questions of relevance to radiotherapy. (1) The proposal to use pulsed rather than continuous irradiation in interstitial brachytherapy. (2) The equivalence of high dose-rate and low dose-rate intracavitary treatments for carcinoma of the cervix. (3) An analysis of equivalent doses for a range of dose-rates in interstitial implants.