PPP03 Presentation Time: 10:48 AM

Purpose

Erectile disfunction (ED) is a common long-term side effect of any prostate cancer treatment. Radiation-related ED is thought to be a vascular process, related to venous occlusion caused by radiation. Low dose-rate (LDR) brachytherapy (BT) is a well-established treatment modality that has an undisputable dose conformality and a theoretical advantage in reducing dose to erectile-related structures when compared to EBRT options. Nevertheless, the role of vessel-sparing LDR-BT technique has not been previously described.

Materials and Methods

PRIAPUS (NCT 04718987) is a prospective, single-arm clinical trial evaluating feasibility and dosimetry associated with a novel LDR BT technique that aims to spare ED-related structures, including the prostatic neurovascular bundles (NVB) contralateral to the index lesion and the penile bulb (Figure 1). The trial planned to accrue 15 patients with intermediate-risk prostate cancer staged with MRI, with clinically significant disease (CSD) contained to one lobe of the prostate. Primary objective was for 70% of patients to achieve acceptable dose to the target structure while sufficiently sparing ED-related structures. Dosimetry was evaluated on a one-month post-implant CT-scan. In order of priority, the trial's dosimetric goals are: target D90% ≥ 140 Gy, urethra D30% < 130%, contralateral NVB median dose ≤ 50 Gy, and penile bulb D10% ≤ 50 Gy. The LDR BT workflow involved a pre-procedural prostate mpMRI for NVB definition, intra-operative use of a deformable image registration algorithm to translate NVB contours from pre-implant MRI to the live ultrasound images, and intraoperative planning using loose 125-Iodine radioactive seeds with a prescription dose of 145 Gy.

Results

Fifteen patients have been consented: one withdrew consent before receiving treatment, one awaits treatment, and 13 have been successfully treated with post-implant dosimetry available for analysis. In the one-month post-procedure scan, the mean target D90% was 153 Gy (SD ± 10 Gy). All patients but one had a target D90% > 140 Gy. The mean urethra D30% was 129% (SD ±10%). The mean contralateral NVB D50% was 60 Gy (SD ± 13 Gy), with 10 of 13 implants failing to meet the pre-specified goal. For comparison, the ipsilateral NVB which was not spared received a mean D50% of 131 Gy (SD ± 33 Gy). The mean penile bulb D10% was 32 Gy (SD ± 13 Gy). Only two patients had a post-implant dosimetry that met all pre-specified criteria, and so this trial's primary dosimetric endpoint will not be met after the last patient receives treatment.

Conclusions

The pre-specified dosimetric goals were found stringent and seldom achievable and future trials with this technique will require relaxation of the contralateral NVB constraint. While the primary dosimetric endpoint was not met, a substantial dose sparing to the contralateral NVB was achieved while maintaining an ablative dose to the subtotal prostate. Figure 1- Mid-gland T2 axial sequence at 1-month post LDR BT implant in a patient with CSD located in the right lobe. Note dose avoidance to the contralateral neurovascular bundle (contoured in yellow). Prescription dose (in pink colour) is delivered to the subtotal prostate (in blue). Yellow and green lines represent 120% and 150% isodoses.