Fractionated high dose-rate versus low dose-rate regimens for intracavitary brachytherapy of the cervix: equivalent regimens for combined brachytherapy and external irradiation.

The conventional treatment for carcinoma of the uterine cervix is a combination of external teletherapy and low dose-rate (LDR) intracavitary brachytherapy. Recently, however, there has been an increasing trend toward the use of high dose-rate (HDR) brachytherapy, in combination with external irradiation. The question is addressed of designing HDR treatments that will produce equivalent results to the more conventional protocols. We argue that for the unique case of radiotherapeutic treatment of carcinoma of the cervix, the criterion for producing an equivalent treatment should be based on the matching of early, not late, effects. In essence, this is because the dose to the tissues at risk for late effects is usually significantly smaller than the prescribed dose. When this effect is factored in with the different shape of dose-response curves for early and late effects, we conclude that, in the majority of cases, late effects will be no worse in a HDR regimen than a LDR regimen, provided that the corresponding doses have been matched to produce equal early effects. We provide a formalism whereby equivalent protocols can be designed for combined "external + HDR brachytherapy" regimens to match current "external + LDR brachytherapy" schedules. Using extensive basic radiobiological in vitro data derived from various cells of human origin, we provide specific examples of equivalent "external + HDR brachytherapy" regimens for 23 current "external + LDR brachytherapy" commonly-used schedules.