THE DOSE-RATE FACTOR IN RADIATION BIOLOGY

The dose-rate effect has been the topic of extensive radiobiological studies and has important implications in radiation therapy and in the field of radiation protection. Three examples will be discussed: two in radiation therapy and one in protection. First, continuous low dose-rate interstitial brachytherapy may be replaced by pulsed brachytherapy, using a single source moving through the catheters of the implant. This strategy, using a modern computer-controlled afterloading device, would allow better dose optimization and result in a considerable cost saving. Radiobiological data have proved useful in defining the pulse length and pulse frequency that is equivalent to continuous low dose-rate. Second, in the intracavitary treatment of carcinoma of the cervix, a few high dose-rate (HDR) fractions on an outpatients basis can replace the low dose-rate (LDR) treatment that requires the patient to be hospitalized for several days. Radiobiological data can be used to estimate the dose levels at HDR that are equivalent to conventional LDR protocols. Third, it is usually assumed in radiation protection that doses accumulated over a period of time at low dose-rate are less effective biologically than the same dose delivered in a single acute exposure. While this may be true for X- or gamma-rays, radiobiological data indicate that neutrons delivered at low dose or in a series of fractions spread out over a period of time may produce more oncogenic transformation than a single acute exposure. This has important implications in radiation protection.