S1220
Clinical - Urology
ESTRO 2026
[0.1%,7.9%] (Figure 2).
Purpose/Objective: Moderate hypofractionation radiotherapy is a standard of care for prostate cancer, although studies have reported higher incidence of early gastrointestinal (GI) toxicity1. Biodegradable peri- rectal spacer has shown to improve rectal dosimetry and reduce GI toxicity from randomized trials2,3, yet its clinical value is underreported in real-world settings. This study evaluates on the dosimetric, clinical, and patient-reported outcomes of rectal spacing in prostate cancer volumetric modulated arc therapy (VMAT). Material/Methods: Consecutive patients with low or intermediate-risk prostate cancer who completed moderate hypofractionation VMAT between January 2022 and July 2025 at a single institution in Hong Kong were included in this retrospective study. Peri-rectal spacer implant was performed before MR simulation for all eligible patients who consented. Patients received 20 fractions of 3Gy over 4 weeks, with cone beam CT guidance. Androgen-deprivation therapy for 6 months was given for unfavourable intermediate-risk. Primary outcomes were rectal dosimetry, and health-related quality of life (HRQOL) measured by the Expanded Prostate Cancer Index Composite (EPIC-26) and The 5- level EQ-5D version (EQ-5D-5L), which were prospectively collected. Clinician-assessed toxicity was graded by Common Toxicity Criteria for Adverse Events (version 5.0). Outcomes of patients who received radiotherapy with- and without peri-rectal spacer were compared. Results: Seventy patients were identified, including 32 with peri-rectal spacer and 38 without. The median prostate volume was 44.9mL (31.6, 63.5 (IQR)) and 50.1mL (30.3, 87.0) for the spacer and no-spacer groups (Table 1). Median follow-up was 15.9 months. Significantly more favourable rectal dosimetry was observed in the spacer group across V42Gy to V60Gy (p<0.0001 for all dose levels) (Figure A), with slightly higher PTV60Gy D99% coverage (p=0.03). The incidence of acute grade 1 GI toxicity was 15.6% and 39.4% for the spacer and no-spacer groups, respectively (p=0.04). No grade 2 or above acute GI toxicity was observed. The cumulative incidence of grade 1 and grade 2 late GI toxicity was 10.3% and 0%, respectively, for the spacer group, vs. 15.2% and 15.2% for the no-spacer group (p=0.07) (Figure B). The proportion of patients with a reduction exceeding the Minimal Clinically Important Difference (MCID) in the EPIC-26 bowel domain was significantly lower in the spacer group at completion of radiotherapy (p=0.002) (Figure C). EQ-5D-5L scores were maintained over time. Changes in IPSS followed similar patterns in both groups (Figure
Conclusion: PTV margins of 3 mm to the prostate and 4 mm to the seminal vesicles and pelvic lymph nodes are adequate to ensure target coverage for patients with prostate cancer treated with oART in our institution. Margin reduction reduces doses to organs-of-interest and has the potential to lower the risk of side effects. NTCP reduction varies between patients. Further studies are needed to designate patients that particularly benefit from margin reduction. References: [1]: Wouter Schaake, Arjen van der Schaaf, Lisanne V. van Dijk, et al. Normal Tissue Complication Probability (NTCP) Models for Late Rectal Bleeding, Stool Frequency and Fecal Incontinence After Radiotherapy in Prostate Cancer Patients. Radiotherapy and Oncology, 119(3):381–87, 2016. Keywords: Normal tissue Digital Poster 2244 Clinical and dosimetric impact of peri-rectal spacer in moderate hypofractionation volumetric modulated arc therapy for prostate cancer Philip Y Wu, Kelvin YC Zheng, William WL Wong, Jeffrey MH Yu, Hing Ming Hung, Ka Ho Tse, Kuen Chan Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong, Hong Kong
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