A proposed figure of merit for the assessment of unscheduled treatment interruptions

There are, as yet, no standard radiobiological methods for devising compensation for unscheduled interruptions to fractionated radiotherapy. For the foreseeable future it is likely that the concept of biologically effective dose (BED) will play an important role in the intercomparison of treatment regimes, and in the examination of the options available for dealing with unscheduled treatment interruptions. However, comparison of the BEDs associated with different treatment options does not provide an intuitively obvious indication of the magnitude of any associated differences in biological effect - an important consideration in the case of those treatments which are designed to deliver near-tolerance doses. This article reviews the implications which derive from this complication, and discusses the desirable properties of possible ''one-number'' treatment scoring systems which could utilize the BEDs of both tumour and the critical normal tissue. One possible form of such a scoring parameter is suggested, and applied to some clinical examples. (author)