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 periodically introduced into the patient (i.e., pulsed) using a remote afterloader. It has been reported that by a careful choice of pulse length and frequency and using the ERD bioeffect dose model, therapeutic advantage (TA) values 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 late-reacting normal tissue ERD. These calculations are based upon a uniform average dose rate in each pulse and equal repair rate constants for both tumor 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 involves a single stepping source. A generalized ERD equation based on the linear quadratic model has been developed to account for the variation in the dose rate and, subsequently, to maximize the TA. Our calculations indicate 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.