S1161
Clinical - Urology
ESTRO 2026
Material/Methods:
70% CR and 30% PR rate. Conclusion:
MRL focal SABR for unifocal intermediate-risk prostate cancer was delivered within acceptable time frames, with transient mild toxicity and minor QOL impacts. PSA, mpMRI and PSMA responses were favourable. These results underline the potential of the MRL approach, justifying further patient accrual and long- term analysis for validation References: 1.Tempany-Afdhal CMC. Focal treatment of prostate cancer: MRI helps guide the way forward. Radiology. 2021;298(3):704-6.2. Ś lusarczyk A, Gurwin A, Barna ś A, et al. Outcomes of Focal Therapy for Localized Prostate Cancer: A Systematic Review and Meta- analysis of Prospective Studies. European urology oncology. 2025.3.Westley RL, Valle LF, Tree AC, Kishan AU. MRI-Guided Radiotherapy for Prostate Cancer: Seeing is Believing. Seminars in radiation oncology. 2024;34(1):45-55.4.Kishan AU, Ma TM, Lamb JM, et al. Magnetic Resonance Imaging–Guided vs Computed Tomography–Guided Stereotactic Body Radiotherapy for Prostate Cancer: The MIRAGE Randomized Clinical Trial. JAMA oncology. 2023;9(3):365-73. Keywords: MRL guided, Adapted, Focal SABR, Prostate cancer Digital Poster 972 Mind the Gap! Does spacer symmetry save rectal doses in Prostate SBRT? Su Xin Ghislaine Lee 1 , Jeffrey Kit Loong Tuan 1,2 , Li Kuan Ashley Ong 1 1 Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore. 2 Duke_NUS, Duke NUS Graduate Medical School, Singapore, Singapore Purpose/Objective: Hydrogel spacer placement between the prostate and rectum has shown to effectively reduce rectal doses during prostate stereotactic body radiotherapy (SBRT). However, the effectiveness of rectal dose reduction is highly dependant on the distribution of the spacer and its ability to separate the prostate from rectum within the perirectal space. Previous studies examined spacer symmetry using binary assessments of spacer presence at the midline and 1–2 cm lateral positions1- 2. This study advances that approach by proposing a symmetry scoring method which quantitatively incorporates lateral spacer thickness relative to the midline thickness. This study aims to correlate spacer symmetry scores with rectal dose reduction, and most importantly, to find the ideal score and spacer positioning which produces the best rectal sparing.
A retrospective study on 95 patients who received prostate SBRT (36.25Gy-37.5Gy in 5 fractions) with hydrogel spacer placement was performed. Spacers were contoured on the planning CT and assessed for midline symmetry. Spacer thickness was measured at prostatic midline, 1cm to the right and left, at the base, midgland and apex, yielding 9 measurements for each patient. Subsequently, a spacer quality metric (SMScore) was developed to score spacer thickness medial-lateral symmetry relative to midline, at each axial plane (Figure 1). In Figure 1b, midline spacer thickness was 0.9cm. Measurements at 1cm right and left of midline was 1.3cm and 0.8cm respectively, within the range of 0.45-1.35cm (0.5x less-1.5x more of 0.9cm), giving a SMScore of 3.A binary logistic regression was performed to ascertain the effects of SMScores and spacer thickness measurements on rectal dose-volume (DV) metrics of D0.03cc ≥ 36Gy, D1cc ≥ 32Gy and D3cc ≥ 29Gy. Results:
The most common SMscore at the base, midgland and apex was 0, 3 and 0 respectively.Regression analysis showed SMScores were not statistically significant in predicting rectal DV metrics, while spacer thickness at midgland and apex was statistically significant (p<0.05) (Table 1). Notably, Apex_ML spacer thickness was most significantly associated with rectum D1cc exceeding the 32Gy threshold (OR: 0.072) and D3cc exceeding 29Gy (OR: 0.064), (p<0.01). Conclusion:
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