HDR versus LDR gynecological brachytherapy revisited.

Despite the obvious breakthrough of high dose-rate (HDR) afterloading systems on the gynecological brachytherapy market, questions still remain regarding the transfer of available expertise gained throughout the last 80 years with low dose-rate (LDR) radium and cesium, especially regarding the conversion of LDR total dose into equivalent HDR dose per fraction and total dose. Calculation of biologically equivalent schedules requires a knowledge of repair capacity and repair kinetics of tumors and normal tissues, both of which influence the biological effect of any radiation dose. The clinical experience with HDR is, however, accumulating and it is acknowledged that the new technique entails an acceptable therapeutic index as compared to the classical LDR. There is thus a state of apparent 'equivalence' between the two treatment modalities. This state is influenced by many factors in which, in contrast to what is frequently claimed, radiobiological factors do not play the most important role. It is probably its high-tech environment which makes HDR an acceptable alternative. Treatment at LDR, indeed, has proven to be quite tolerant to a lack of absolute precision, something that would be disastrous with HDR techniques. Because HDR intracavitary brachytherapy has not been compared in controlled trials with the best existing LDR brachytherapy, but only retrospectively with heterogeneous LDR clinical data, it cannot be claimed to be equivalent, but simply feasible.