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

The continued reliance upon the traditional biologically effective dose (BED_T) formalism of BED_T=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_T 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_T.

BED was recalculated with a model accounting for iSLDR, using time-averaged uniform dose rate (BED_g1) patterns and time-variable dose rate (BED_gss) 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.

BED_g1 and BED_gss was/were lower than BED_T by 16.9% and 11.1%-21.1%, respectively, for 40700 U. For 17470 U, BED_g1 and BED_gss was/were lower than BED_T by 32.5% and 21.5%-37.1%, respectively. For 61050 U, BED_g1 and BED_gss was/were lower than BED_T by 11.9% and 7.8%-15.3%, respectively. BED_gss was most dependent on pulse spacing with milder dependence on pulse onset time. BED_g1 served as a lower bound approximation of BED_gss for fast effective delivery time.

Even for points with the same calculated dose, the biological dose was significantly reduced by iSLDR (as much as 37.1%). While BED_gss explicitly addressed the temporally-variable dose rate inherent to a stepping-source delivery, calculations were cumbersome. Under certain conditions, BED_g1 may serve as an approachable method to quickly assess "worst-case scenario" BED.