Single-Dose Image-Guided Radiotherapy With Urethral Sparing and DIL Boost for Intermediate-Risk Prostate Cancer (PROSINT II)

This prospective single-arm phase II study evaluates the safety, feasibility, quality-of-life effects, PSA kinetics, imaging response, and clinical outcomes of definitive ultra-high dose single-fraction external beam radiation therapy in patients with biopsy-proven NCCN intermediate-risk localized adenocarcinoma of the prostate. All eligible patients receive image-guided volumetric modulated arc radiotherapy with urethral sparing and organ-motion mitigation. Treatment consists of 24 Gy in one fraction to the whole prostate gland and proximal seminal vesicles. Patients with NCCN unfavorable intermediate-risk disease and an imaging-defined dominant intraprostatic lesion may receive a PSMA PET/CT-guided simultaneous integrated boost to the dominant intraprostatic lesion in sequential dose-escalation cohorts, up to 30 Gy, while maintaining protocol-defined organ-at-risk constraints. A rectal balloon with air filling is used for prostate target immobilization and anatomical reproducibility, and a urethral catheter loaded with beacon transponders is used to identify the urethra, support urethral sparing, and enable online target tracking. Toxicity is assessed using CTCAE v4.0, and patient-reported outcomes are assessed using EPIC-26, IPSS, and IIEF questionnaires. PSA is measured at protocol-defined follow-up visits. Multiparametric MRI is performed at baseline and at 12 and 24 months after treatment. Participants are followed for a minimum of 5 years.