Dose differentiated high-dose-rate prostate brachytherapy: a feasibility assessment of MRI-guided dose escalation to dominant intra-prostatic lesions

PURPOSE: Prostate brachytherapy is routinely performed with trans-rectal ultrasound (TRUS)- or computed tomography (CT)-based planning that cannot delineate dominant intra-prostatic lesions (DILs). In contrast, magnetic resonance imaging (MRI)-based planning allows for more accurate DIL delineation and dose escalation. This study assessed the maximum achievable dose escalation to DILs. MATERIAL AND METHODS: We retrospectively identified 24 patients treated with high-dose-rate (HDR) prostate brachytherapy boost (15 Gy in 1 fraction). All patients had a pre-treatment prostate MRI with 1-3 DILs. MRIs were used to delineate DILs and were co-registered to TRUS intra-procedure. Treatment plans were experimentally re-optimized to escalate DIL dose. Dosimetric indices from the original and re-optimized plans were compared using two-tailed paired t-test. Re-optimized plans were deemed acceptable if they achieved all of the following criteria: prostate D(90) > 100%, prostate V(100) > 90%, urethra D(10) < 118%, rectum V(80) < 0.5 cc, bladder D(1cc) < 75%, or if they did not exceed organs at risk (OARs) doses of the original plan. RESULTS: The mean DIL D(90) was significantly increased from 134% of the prescription dose on the original plans to 154% on the re-optimized plans. The mean urethra D(10) and mean bladder D(1cc) were significantly reduced from 123% to 117% and from 72% to 65%, respectively. Prostate D(90) was reduced from 106% to 102%, and prostate V(100) was reduced from 93% to 91%. CONCLUSIONS: We re-optimized HDR brachytherapy plans to escalate DILs dose to a mean D(90) of > 150% while maintaining favorable prostate coverage and OARs doses. We propose DIL D(90) dose of > 150% (22.5 Gy) as an achievable goal.