Main Session
Sep
29
PQA 06 - Genitourinary Cancer, Gynecological Cancer, and Health Care Access and Engagement
3400 - Prospective Study of Rectal Spacer Application with Adaptive MR-Guided SBRT in cT3a-T3b Prostate Cancer
Presenter(s)
Darren Poon, MD - The Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong
D. Poon1, O. Wong2, J. Yuan3, M. Y. Tse2, B. Yang4, S. T. Chiu5, G. Chiu6, and S. K. Yu4; 1The Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong, 2Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong, 3HKSH Medical Group Ltd, Happy Valley, Hong Kong, 4Medical Physics Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, 5Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong, 6Department of Radiotherapy, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong
Purpose/Objective(s):
Rectal spacers reduce rectal dose and gastrointestinal (GI) toxicity in localized prostate radiotherapy; however, patients with cT3a–T3b disease are commonly excluded from spacer trials due to concerns regarding extracapsular extension (ECE), seminal vesicle invasion (SVI), tumor involvement at the prostate–rectum interface, and potential compromised target coverage. Prospective data in this population remain limited. We hypothesized that rectal spacer application with adaptive MR-guided stereotactic body radiotherapy (MRgSBRT) in cT3a–T3b prostate cancer is feasible, and preserves target coverage without increasing treatment-related toxicity.Materials/Methods:
This prospective cohort included consecutive cT3a–T3b prostate cancer patients treated between 09/2020–04/2025 with 5-fraction adaptive MRgSBRT (40 Gy) following hydrogel or hyaluronic acid spacer placement. Optional intraprostatic boost and elective whole-pelvic irradiation were permitted. Spacer geometry was quantified on simulation MRI, including midline and bilateral prostate–rectum separation; differences were assessed using Kruskal–Wallis test. Dosimetric endpoints included PTV V100% and rectal V95%, V90%, and V50%. Acute (1–3 months) and late (>4 months) toxicities were graded using CTCAE v5.0. Quality of life was assessed using EPIC; minimally important difference (MID) was defined as =4–6 points. Survival outcomes were estimated using Kaplan–Meier analysis.Results:
29 patients were included (mean age 76±8 years; median follow-up 18.7 months): ECE (n=19), ECE with SVI (n=6), and isolated SVI (n=4). All spacer placements were successful without procedural complications. Mean spacer volume was 11.0±2.3 mL. Mean midline prostate–rectum separation was 9.1±2.5 mm, with no significant difference between midline and lateral measurements (p>0.05), confirming symmetric distribution. Target coverage was preserved (mean PTV V100%=98.6%). Rectal dose constraints were achieved in all patients (mean V95%=0.35 cc; V90%=0.79%; V50%=24.5%), demonstrating effective rectal sparing without compromising coverage. No acute or late grade =2 GI toxicity occurred. At 2 years, EPIC bowel function and bother scores showed no clinically meaningful decline and met MID stability criteria. Among locally advanced patients (n=15), 2-year biochemical recurrence-free and metastasis-free survival were both 100%.Conclusion:
In this prospective cohort, rectal spacer application with adaptive MRgSBRT in selected cT3a–T3b prostate cancer was feasible and well tolerated, providing consistent prostate–rectum separation, favorable rectal dosimetry, and preserved target coverage without clinically significant gastrointestinal toxicity. These findings provide prospective safety and dosimetric evidence supporting further evaluation of spacer use in locally advanced prostate cancer, a population historically underrepresented in clinical trials.