Derivation of isoeffect dose rate for low-dose-rate brachytherapy and external beam irradiation

Purpose: The combination of external beam irradiation and brachytherapy has been used effectively in the management of many malignancies. Brachytherap y dose is typically prescribed to an isodose rate line, from which the impl ant duration is derived. In this study, the linear-quadratic model is used to derive the brachytherapy dose rate at which biological effectiveness is equivalent to that of external beam irradiation. Methods and Materials: Relative effectiveness per unit dose (RE) for brachy therapy was based on Dale's formalism, Isoeffect dose rate, defined as the brachytherapy dose rate at which the biological effectiveness is equivalent to that of external beam irradiation, was derived, Results: The functional dependencies of brachytherapy RE on dose rate, alph a/beta ratio, and implant duration were investigated. The isoeffect dose ra te depends only on the dose per fraction, sublethal damage repair (SLDR) co nstant, and the implant duration. The isoeffect dose rate does not depend o n alpha/beta ratio. For sufficiently long implant duration >10-15 hours, th e value for isoeffect dose rate approaches a constant value around 40 to 50 cGy/hr. Conclusion: The isoeffect dose rate may be useful in treatment planning and optimization for low-dose-rate (LDR) brachytherapy, especially when brachy therapy is used in combination with external beam irradiation. (C) 1999 Els evier Science Inc.