3179 - Prostate-Specific Membrane Antigen-Directed Focal Stereotactic Radiation Therapy in Two Fractions for Locally Recurrent Prostate Cancer after Definitive Radiation Therapy

L. Hammer¹, R. Lewin¹, J. Mattout¹, I. Bar Orian¹, M. ben-Ayun¹, L. Tsvang¹, S. Dubinski¹, I. Weiss¹, Y. Lawrence^1,2, and Z. Symon^1,2; ¹Radiation Oncology Department, Sheba Medical Center, Ramat Gan, Israel, ²Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel

Purpose/Objective(s): Five fraction salvage stereotactic body radiation therapy (SBRT) is a Delphi consensus recommendation for prostate re-irradiation, yet remains debatable and underutilized, and is associated with higher risk of treatment-related adverse events compared to primary radiotherapy. We hypothesized that intensification of hypo-fractionation delivering 2 fractions of 9 GY would be advantageous and here report the feasibility, toxicity, and preliminary outcomes of 68Ga-prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) directed salvage focal re-irradiation with this regimen. Materials/

Methods: From July 2022 to November 2023, 17 Men with isolated prostate cancer recurrence after definitive radiation were treated at single institution. All patients underwent 68Ga-PSMA PET/CT following biochemical recurrence by the phoenix criteria or a repeated PSA increase. Confirmation of recurrence was performed by prostate biopsy and multiparametric MRI in 53% and 35% of patients, respectively. Focal SBRT of 18 Gy in two fractions was delivered to the PSMA-avid disease and a 3-5mm margin in the prostate on alternate days with image guidance by fiducial markers. Placement of hydrogel spacer (29%) or endorectal balloon (71%) was mandated. Androgen deprivation therapy was given concurrently in 88% of patients for a median duration of six months (range, 4-21). Toxicity was assessed according to Common Terminology Criteria for Adverse Events version 5.
Results: Median follow-up was 11.9 months (range, 3-18) with a median time between primary radiation treatment and salvage SBRT of 4.3 years (range, 0.6- 15.6). Median age was 73.1 (range, 66-89) and Prostate-specific antigen (PSA) level at reirradiation was 1.87 ng/ml (range, 0.02-7.1). At the end of follow up only one patient had a biochemical recurrence, which was confirmed by 68Ga-PSMA PET/CT. No patient developed acute or late gastrointestinal toxicity, and four patients (23.5%) did not have any adverse events. Rates of acute genitourinary (GU) toxicity grade 1 and 2 were 41.2% and 11.8%, respectively. Late grade 1 and 2 GU toxicity occurred in 41% and 35%, with cystitis as the leading toxicity followed by urinary incontinence. No grade 3 or 4 toxicities were reported.
Conclusion: With a short follow-up, PSMA-directed salvage focal reirradiation to the prostate using SBRT of 18 Gy in two fractions is feasible and has acceptable toxicity. No gastrointestinal toxicity was reported, as well as no high grade toxicity of any type, with very favorable short term biochemical control in a selected cohort of patients.