Po79

Purpose Promising results have been demonstrated using single fraction external beam for treatment of prostate cancer. Due to inherent differences of delivery between external beam and brachytherapy, the feasibility of replicating the external beam dosimetry in brachytherapy is investigated. Materials and Methods A retrospective study was performed using datasets of 120 HDR prostate patients from 6 different physicians in a single institution. The original plans were prescribed to 15Gy (with EBRT to follow). No changes were made to catheter placement, dwell positions, or contours. An in-house software, a linear optimizer with bounded and unbounded constraints, was used to elevate the dose to the entire gland to 24 Gy with no manual manipulation of the optimized plan/dwell times. The resulting plans were compared with published results of single fraction external beam plans to a dose of 24Gy, using the same dosimetric constraints [1]. Prostate D50%, Dmean, D95%, D2%, V24Gy, and V21.6Gy were reported, as well as OAR constraints D1cc for the rectum and urethra. Results Brachytherapy vs external beam prostate median metrics were 31.4 vs 24.5 Gy for Dmean; 22.1 vs 22.1 Gy for D95%; 79.8 vs 84.8% for V24Gy; 96.2 vs 96.2% for V21.6Gy. D1cc median metrics were 10.6 vs 18.5Gy for urethra and 18.2 vs 18.8Gy for rectum. Conclusion Dose escalation to the prostate was feasible and achieved similar results to external beam single fraction radiotherapy in terms of target coverage and OAR constraints. Due to the inherent differences of radiation delivery, the dose distributions differ primarily in hotspots and non-homogeneity that could lead to clinical differences. Delivery uncertainty is believed to be lower in brachytherapy, resulting in shrinking of margins and more accurate placement and delivery of hotspots, if desired. The retrospective study was based upon fixed prior implant geometries, therefore further work to investigate the placement of needles to further optimize dose conformality is ongoing. [1] Greco C et al. Safety and Efficacy of Virtual Prostatectomy With Single-Dose Radiotherapy in Patients With Intermediate-Risk Prostate Cancer: Results From the PROSINT Phase 2 Randomized Clinical Trial. JAMA Oncol. 2021 May 1;7(5):700-708. Promising results have been demonstrated using single fraction external beam for treatment of prostate cancer. Due to inherent differences of delivery between external beam and brachytherapy, the feasibility of replicating the external beam dosimetry in brachytherapy is investigated. A retrospective study was performed using datasets of 120 HDR prostate patients from 6 different physicians in a single institution. The original plans were prescribed to 15Gy (with EBRT to follow). No changes were made to catheter placement, dwell positions, or contours. An in-house software, a linear optimizer with bounded and unbounded constraints, was used to elevate the dose to the entire gland to 24 Gy with no manual manipulation of the optimized plan/dwell times. The resulting plans were compared with published results of single fraction external beam plans to a dose of 24Gy, using the same dosimetric constraints [1]. Prostate D50%, Dmean, D95%, D2%, V24Gy, and V21.6Gy were reported, as well as OAR constraints D1cc for the rectum and urethra. Brachytherapy vs external beam prostate median metrics were 31.4 vs 24.5 Gy for Dmean; 22.1 vs 22.1 Gy for D95%; 79.8 vs 84.8% for V24Gy; 96.2 vs 96.2% for V21.6Gy. D1cc median metrics were 10.6 vs 18.5Gy for urethra and 18.2 vs 18.8Gy for rectum. Dose escalation to the prostate was feasible and achieved similar results to external beam single fraction radiotherapy in terms of target coverage and OAR constraints. Due to the inherent differences of radiation delivery, the dose distributions differ primarily in hotspots and non-homogeneity that could lead to clinical differences. Delivery uncertainty is believed to be lower in brachytherapy, resulting in shrinking of margins and more accurate placement and delivery of hotspots, if desired. The retrospective study was based upon fixed prior implant geometries, therefore further work to investigate the placement of needles to further optimize dose conformality is ongoing. [1] Greco C et al. Safety and Efficacy of Virtual Prostatectomy With Single-Dose Radiotherapy in Patients With Intermediate-Risk Prostate Cancer: Results From the PROSINT Phase 2 Randomized Clinical Trial. JAMA Oncol. 2021 May 1;7(5):700-708.