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.