3218 - Sparing the Urethra, Pudendal Arteries and Rectum during Stereotactic Ablative Body Radiotherapy (RT) for Prostate Cancer (SUPR-SABR): Acute Toxicity and Dosimetric Analysis of an Ongoing Trial

Purpose/Objective(s): SUPR-SABR is a phase II trial that is evaluating SABR that spares the urethra, pudendal arteries (PAs), and rectum in men with prostate cancer. We aim to report the dosimetry and acute toxicity in men treated with the SUPR-SABR technique on this ongoing trial.

Materials/Methods: Following informed consent, from June 2023 through September 2024, 22 patients with low- and intermediate-risk prostate cancer were treated with a linac SABR platform. Inclusion criteria include gland size <120 mL and International Prostate Symptom Score (IPSS) score =20. Exclusion criteria include prior prostate cancer treatment or prior pelvic RT. The 40 Gy planning treatment volume (PTV) includes the prostate with a 5 mm expansion except posteriorly. The rectum is excluded from the PTV 40 Gy. The PTV 36.25 Gy volume is the prostate with 5 mm expansion except 3 mm posteriorly. The urethral planning risk volume (PRV) is excluded from the PTVs. All patients are treated with rectal spacer gel, endorectal balloons, fiducial matching, and three-dimensional IGRT. The urethra is delineated using a foley catheter placed during simulation. Acute genitourinary (GU) and gastrointestinal (GI) toxicity is defined as any treatment related (attributions of possible, probable, definite) incidence that occurs during & up to 30 days after RT and is scored with CTCAE version 5.0. The primary endpoint is change in Expanded Prostate Cancer Index Composite (EPIC) GU and GI domain scores at 12 months and is not mature for analysis. Pre-specified secondary endpoints (24 month EPIC scores, PSA failure, late toxicity, change in Sexual Health Inventory for Men (SHIM) scores) are also not mature for analysis. Target sample size is 36 participants.

Results: The median age was 71.3 years (Q1 64.1, Q3 74.5) with median PSA and Gleason score of 6.8 (Q1 4.6, Q3 8.5) and 7 (range, 6-7) respectively. 8 (36.4%) patients received androgen deprivation therapy, the median prostate gland size was 32.8 cc (Q1 26.2, Q3 47.0) with a median pretreatment IPSS score of 6 (Q1 2.0, Q3 12.8). No acute grade=3 adverse event. Participant-level incidence of grade =2 GU and GI AEs were 41% (9/22) and 5% (1/22), respectively. Initial dosimetry is reported in the Table.

Conclusion: The SUPR-SABR trial and technique limits dose to the urethra, PAs, and rectum. Late toxicity, biochemical control, and patient reported outcomes are awaited. (NCT04505267)