Radiobiological evaluation of the stepping-source effect in single-fraction monotherapy high-dose-rate prostate brachytherapy.

Introduction: The continued reliance upon the traditional biologically effective dose (BED) formalism of BED=nd(1+d/(α/β)) may be one possible contributor to the poor clinical outcomes observed with single-fraction 19-20 Gy prescriptions in prostate high-dose-rate (HDR) brachytherapy because BED does not consider intrafraction sublethal damage repair (iSLDR). This, along with low α/β and repair half-times comparable to delivery time, could reduce the biological effect predicted using BED.

Methods And Materials: BED was recalculated with a model accounting for iSLDR, using time-averaged uniform dose rate (BED) patterns and time-variable dose rate (BED) patterns inherent to stepping-source delivery. An assortment of two-pulse delivery sequences assuming 19 Gy in 972 s was analyzed. Calculations were repeated for 17470 and 61050 U to investigate source strength dependence.

Results: BED and BED was/were lower than BED by 16.9% and 11.1%-21.1%, respectively, for 40700 U. For 17470 U, BED and BED was/were lower than BED by 32.5% and 21.5%-37.1%, respectively. For 61050 U, BED and BED was/were lower than BED by 11.9% and 7.8%-15.3%, respectively. BED was most dependent on pulse spacing with milder dependence on pulse onset time. BED served as a lower bound approximation of BED for fast effective delivery time.

Conclusions: Even for points with the same calculated dose, the biological dose was significantly reduced by iSLDR (as much as 37.1%). While BED explicitly addressed the temporally-variable dose rate inherent to a stepping-source delivery, calculations were cumbersome. Under certain conditions, BED may serve as an approachable method to quickly assess "worst-case scenario" BED.