Novel low dose rate brachytherapy with focal sparing of neurovascular bundle: Report on the primary outcome from the PRIAPUS trial.

Purpose: Erectile dysfunction (ED) is a common side effect of any prostate cancer treatment and the role of vessel-sparing low dose rate brachytherapy (LDR-BT) technique has not been previously described.

Materials and methods: PRIAPUS (NCT04718987) is a prospective, single-arm clinical trial evaluating feasibility of a novel LDR BT technique designed to spare the prostatic neurovascular bundles (NVB) contralateral to the index lesion. Intermediate-risk prostate cancer patients with clinically significant disease contained to one lobe of the prostate were enrolled. Primary objective was for 70% of patients to achieve acceptable dose to CTV while sufficiently sparing ED-related structures. Dosimetry was evaluated on a 1-month postimplant CT-scan.

Results: Fifteen patients have been consented with 14 patients treated on trial. In the 1-month postprocedure scan, the mean CTV D90% was 152 Gy (SD ± 10.7 Gy). All patients but two had a CTV D90% >140 Gy. The mean urethra D30% was 129% (SD ± 9%). The mean contralateral NVB D50% was 60.8 Gy (SD ± 12.1 Gy), with 11 of 14 implants failing to meet the prespecified goal. The ipsilateral NVB which was not spared received a mean D50% of 128 Gy (SD ± 32 Gy). The mean penile bulb D10% was 31 Gy (SD ± 13 Gy). Only 2 patients had a postimplant dosimetry that met all prespecified criteria.

Conclusions: A novel LDR BT technique is capable of drastically reduce dose to the cNVB, although this reduction did not meet the stringent dose constraints specified in this trial.