Isoeffect relationships for fractionated biologically targeted radiotherapy.

Abstract

This paper describes a method of analysis of the biological effects on normal tissues of fractionated administrations of biologically targeted radiotherapy (BTR). The linear-quadratic (LQ) model as extended by Dale [2] is used to consider the case in which administrations may be separated by time gaps down to the order of a single day. It is assumed that the pharmacokinetics of clearance are linear and that dose-rate profiles in organs are simple exponential decays. The method adopted is to calculate the extrapolated response doses (ERDs) for individual time periods of the treatment between one administration and the next (assuming complete recovery between periods) and additional components which are corrections for incomplete recovery between these time periods. The overall ERD for the course of administrations is given by the sum of these factors. No account is taken of cellular repopulation. As it is likely that fractionated biologically targeted radiotherapy (BTR) will be used in practice, this subject is of clinical relevance. The method is illustrated by a numerical example.