Feasibility and Outcomes of Perirectal Spacer Implantation in Previously Treated Prostate Cancer Patients Undergoing Salvage Proton Therapy Radiation.

Purpose: Treatment of recurrent prostate cancer following prior therapy is challenging due to the risk of toxicities to previously irradiated normal tissues, particularly the rectum. Proton therapy offers a conformal approach that may reduce this risk. Perirectal spacers have been shown to further minimize rectal dose in primary treatment, but the ability to be placed and their value in the retreatment setting have not been thoroughly evaluated to date. The purpose of this study was to evaluate feasibility of rectal spacer placement after prior pelvic radiation or other local treatments for prostate cancer. Methods and Materials: We conducted a retrospective review of 30 consecutive patients with biopsy-confirmed, locally recurrent prostate adenocarcinoma treated with salvage proton therapy at the New York Proton Center (2020-2024). All patients had prior local therapies including high intensity focused ultrasound (HIFU), brachytherapy (BT), external beam radiation therapy (EBRT) or combination BT and EBRT, and underwent perirectal spacer placement (SpaceOAR or Barrigel). Spacer feasibility, maximum and minimum separation distances, rectal symmetry, rectal Dmax (% of prescription dose), and gastrointestinal toxicities were assessed. Toxicity was graded using Common Terminology Criteria for Adverse Events, version 5.0. Results: Median patient age was 74 years, and the median time from prior therapy was 102 months. Spacer placement was successfully completed in 28 of 30 (93%) patients. Two placements were aborted due to excessive fibrosis. The median maximum and minimum midaxial anterior-posterior separations were 10.8 mm and 3.5 mm, respectively. Median rectal Dmax was 91.55% (range, 44%-102.4%). Seven patients had Dmax >100%, including 3 with prior HIFU. At a median follow-up of 12.5 months, gastrointestinal toxicities were limited to grade 1 diarrhea (n=3) No grade ≥ 2 toxicities or procedural complications were reported. Conclusions: This is the first reported study evaluating the feasibility of perirectal spacer implantation in men with prior treatment to the pelvis, undergoing salvage radiation with protons. Our report shows perirectal spacer implantation is feasible and safe in patients undergoing salvage proton therapy, including those with prior EBRT or BT. Although technically more challenging in HIFU-treated patients, successful placement was achieved in the majority, with minimal toxicity and favorable dosimetric outcomes, although longer follow-up is needed.