Mean inactivation dose (D). A critical analysis of a neglected parameter in radiotherapy

By predicting treatment outcome to radiotherapy from in vitro radiobiological parameters, not only individual patient treatments can be tailored, but also new promising treatment protocols can be tried in patients in whom unfavorable outcome is predicted. In this respect, choosing the right parameter can be very important. Unlike D0 and N which provide information of the distal part of the survival curve, mean inactivation dose (D) estimates overall radiosensitivity. However, the parameters reflecting the response at the clinically relevant low-dose region are neglected in the literature. In a literature survey of 98 papers in which survival curves or D0/N were used, only in 2D was used. In 21 papers the D0/N values were important in drawing conclusions. By calculating D in 3 of these 21 papers, we show that the conclusion drawn may be altered with the use of D. The importance of 'low-dose-region-parameters' is reviewed.