The dose-rate effect in interstitial brachytherapy: a controversy resolved.

Low-dose-rate interstitial implants generally involve a range of dose rates varying from about 0.3 to 1.5 Gy/h. It has been a matter of some debate as to whether such a variation in dose rate has any impact on clinical results, i.e. whether implant treatment time needs to be taken into account when prescribing the treatment dose. In particular, Paterson and Ellis both published data based on clinical experience suggesting that the prescribed dose should vary with overall treatment time, whereas the Paris school suggested that isoeffect dose should not be varied for treatment times between 3 and 8 days. Experimental radiobiological data obtained in vitro and in vivo imply a significant change in biological effectiveness over this range of dose rates with a greater variation for normal-tissue late effects than for tumour control. We show, based on radiological considerations and model calculations, that the disagreement between the two schools in their recommendations for generating isoeffect doses for different treatment times was a result of two factors. First, the Paterson recommendation was unequivocally based on matching limiting late effects, whereas the Paris system recommendations were based on an analysis of a combination of late and early effects. Second, the Paterson recommendations were based on a dosimetry system in which the dose rate does not change greatly with tumour volume, whereas the Paris recommendations were based on a dosimetric system in which dose rate is correlated with tumour volume, which in turn is correlated with changes in both tumour control and, especially, necrosis.