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Biologically effective dose using reciprocal repair for varying fraction doses and fraction intervals

¹ Regional Medical Physics Department, Newcastle General Hospital, Newcastle upon Tyne and, ² Northern Centre for Cancer Treatment, Newcastle upon Tyne, UK

Correspondence: Andrew Buckle, Medical Physics, Churchill Hospital, Old Road, Oxford OX3 7LJ, UK. E-mail: andy.buckle{at}orh.nhs.uk

Hyperfractionated regimes are being used more frequently; this has created a clinical need for more sophisticated biologically effective dose (BED) calculations. A formula is given for calculating BED assuming reciprocal repair in the case of changing dose per fraction and changing interfraction interval. Example applications are given for hyperfractionated schedules. It is shown that the formula is useful for calculating isoeffective schedules for a treatment with unplanned gaps and for comparing regimes in trial design. This work should be of value for comparing risk across accelerated schedules when an organ of the central nervous system is at risk. Uncertainties in choice of parameter and model are discussed: the bi-exponential model of repair is invoked for this purpose.