Radiation Dose-Rate: A Factor of Importance in Radiobiology and Radiotherapy

A wide range of dose-rates has been used in radiobiology or radiotherapy, extending from a few rads per day to thousands of rads in a fraction of a second. At ultra-high dose-rates (pulses of micro or nanoseconds) a clear dose-rate effect has been demonstrated for bacteria, but is less certain for mammalian cells; these doserates have no certain application in radiotherapy at the present time. The principal dose-rate effect is observed between 100 rads/minute and 10 rads/hour; the cell-killing effect of X or γ rays decreases continuously as the dose-rate decreases throughout this range, and may be explained readily in terms of the repair of sub-lethal damage taking place during the irradiation. At lower dose-rates cell proliferation continues during the irradiation, and the ultimate outcome is a complex function of cellular radiosensitivity, dose/cell cycle and tissue adaptability. In clinical radiotherapy, the traditional pattern of high dose-rate for external beam therapy and low dose-rate for intracavitary and interstitial therapy has been challenged in recent years. The Cathetron has made possible the use of a high dose-rate fractionated regime for intracavitary treatment of carcinoma of the cervix and for surface mould applications. Conversely, beam therapy at low dose-rate (100 rads/hour) has been proposed and introduced recently, in an attempt to combine the, allegedly, superior results of radium with the safety and convenience of a beam therapy set-up.