Pulsed brachytherapy: A formalism to account for the variation in dose rate of the stepping source

Pulsed brachytherapy is an endeavor to mimic low dose rate (LDR) treatments using a single higher activity source (a medium dose rate) that is periodi cally introduced into the patient (i.e., pulsed) using a remote afterloader . It has been reported that by a careful choice of pulse length and frequen cy and using the ERD bioeffect dose model, therapeutic advantage (TA) value s slightly less than unity can be achieved where TA has been defined as the ratio of tumor ERD for PB to tumor ERD for LDR treatments for constant lat e-reacting normal tissue ERD. These calculations are based upon a uniform a verage dose rate in each pulse and equal repair rate constants for both tum or and normal tissue. In this paper, it is demonstrated that TAs of greater than 1 might be possible, depending upon the repair rate constants assumed for the tissues involved. Furthermore, for PB treatments the dose rate at a point of interest during each pulse is not uniform, since the treatment i nvolves a single stepping source. A generalized ERD equation based on the l inear quadratic model has been developed to account for the variation in th e dose rate and, subsequently, to maximize the TA. Our calculations indicat e that PB performed with 40 pulses in 120 hours with an irradiation time of 30 minutes per pulse with a delay time of two and a half hours is the best replacement for a LDR treatment that delivers 60 Gy in 120 hours. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)02802-3].