Preliminary evaluation of ProSpare®, a rectal obturator for prostate bed radiotherapy within the randomised multicentre POPS trial (NCT02978014)

Background and purpose The introduction of volumetric modulated arc therapy (VMAT) has enabled dosimetric improvements during prostate bed (PB) radiotherapy; however, the planning target volume (PTV) margins remain substantially greater than for prostate radiotherapy. New approaches are required to further optimise rectal dose and minimise target and organ motion. ProSpare ® , a self-inserted rectal obturator designed to facilitate daily image guidance, may enhance radiotherapy accuracy and reduce treatment-related morbidity. Materials and methods Patients scheduled to receive adjuvant or salvage PB radiotherapy after radical prostatectomy were randomised to either standard-of-care (SOC) PB-IMRT using centre-standard PTV margins and verification protocol (SOC) or PB-IMRT with daily ProSpare ® -guided radiotherapy using a reduced 5-mm posterior margin. Dose and fractionation schedules were selected by each centre according to protocol recommendations. The primary endpoint is patient-reported EPIC bowel function and bother at 2 years. Secondary endpoints include acute Radiation Therapy Oncology Group (RTOG) and CTCAE toxicity, patient-reported outcomes, device tolerability, and anorectal dosimetry. We report the results from the prespecified analysis of these secondary endpoints. Results Between 2017 and 2023, 162 patients were recruited; 158 began radiotherapy and were evaluable at 18 weeks. Acute toxicity data were available for 146 patients (SOC 76/81, ProSpare 70/77). Compared with SOC, ProSpare ® was associated with lower 18-week RTOG ≥grade 1 gastrointestinal (GI) and genitourinary (GU) toxicity (39% vs. 31% and 37% vs. 29%, respectively). The incidence of worst CTCAE grade 2 GI and GU toxicity was