The influence of the number of fractions and bi-exponential repair kinetics on biological equivalence in pulsed brachytherapy

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Abstract
A linear-quadratic radiobiological model incorporating single or bi-exponential repair kinetics has been used to show the following and other features when a continuous low dose rate (CLDR) 70 Gy/140 h brachytherapy protocol is replaced by a radiobiologically equivalent pulsed dose rate (PDR) system using 140 fractions for reasons of dosage homogeneity. (1) For equivalent effects in late-reacting tissues, the PDR dose (at 5 or 0.05 Gy min−1) × 1 h intervals needs to be reduced by up to only 3%. Progressively further reductions in dose are required when fewer larger fractions are used. (2) When equivalence using pulsed doses is achieved for one normal tissue type, and extrapolated response doses (ERD) are calculated for other tissue types in the irradiated volume, values of the ERD remain within 5% of each other using the above PDR protocol and associated parameters. (3) For tumours with α/β=10 Gy and a single repair halftime of 0.1–1.0 h, there is no significant loss of therapeutic benefit using ...