Bed-time charts and their application to the problems of interruptions in external beam radiotherapy treatments

Purpose: The use of radiobiological modelling to examine the likely consequ ences of interruptions to radiotherapy schedules and to assess various comp ensatory measures, Methods and Materials: An effect-time graphical display the BED-time chart, has been developed using the linear-quadratic (LQ) model. This is used to examine the effects on tumour and normal tissues of treatment interruption scenarios representative of clinical situations, The mathematical criteria governing successful salvage have also been drafted and applied to typical situations. Results: The successful salvage of an interrupted treatment is dependent on a number of interacting factors and the method presented here can be used to examine the trade-offs that exist, Although the mathematics may be compl ex, it is shown that the dilemmas posed by an interrupted treatment may be more easily appreciated with reference to BED-time charts. These may theref ore have a useful role as a teaching aid for portraying a wider variety of radiotherapy problems and also in the documentation of interruptions to tre atment and the measures taken to compensate for them, Conclusions: Interruptions to radiotherapy regimes are undesirable and comp ensatory measures need to be initiated as soon as possible after the gap, w ith a view to completing the amended treatment within the originally prescr ibed treatment time. Adequate compensation is particularly difficult for lo ng gaps and gaps which occur towards the end of the scheduled treatment. Mo delling exercises can help establish guidelines on the available windows of opportunity. (C) 1999 Elsevier Science Inc.