S154
Brachytherapy - Urology
ESTRO 2026
Digital Poster 606
with LDR brachytherapy. These findings could provide further rationale for targeted dose escalation in prostate SABR. Material/Methods: We examined a cohort of 606 patients treated with LDR brachytherapy (145Gy monotherapy n=546; 110Gy combination n=60) in a single institution from 2009-2020 (13% low, 80% intermediate, 7% NCCN high-risk). The prostate sector(s) in which the DIL was located, was/were identified following review of the diagnostic MRI (n=599) and pathology report (n=606). Outcomes were blinded during this process. The prostate was divided into 12 sectors; 3 equal thirds (base/midgland/apex), and 4 axial sectors by two separate methods; plus-shape (“+”) and cross-shape (“x”). Kaplan-Meier curves were generated with the log- rank test performed to assess for differences in freedom from BCF rates. Results: Five-year freedom from BCF for the entire cohort was 89.6%. Relative to the global prostate gland (D90 108.0%, IQR 101.8-114.3%), the anterior base sectors received the lowest median D90 (85.9%, IQR 76.6- 95.6%). Patients with DILs in these sectors experienced the lowest rates of freedom from BCF (anterior right base 76.7%, HR of failure 5.4 (95% CI 1.7-17.0), p=0.004 and anterior left base 49.1%, HR of failure 9.6 (95% CI 2.9-31.3), p=0.0002), relative to the overall cohort. The posterior mid-gland received the highest median D90 (141.8% (IQR 128.3-155.5%)) and DILs positioned in this sector had the greatest freedom from BCF (96.8%, HR 0.4 of failure (95% CI = 0.2-0.7) p=0.002), relative to the overall cohort. BCF was 15 times more likely to occur when DILs were positioned at the base of the prostate where doses were lowest, compared to the posterior mid-gland. Conclusion: Our study demonstrates that both the location of, and dose delivered to, the DIL are strongly associated with BCF. As most local prostate cancer recurrences occur at, or near, the original DIL, these results provide further evidence that focal dose escalation may improve prostate cancer outcomes. These findings are relevant to future prostate SABR clinical trials. Keywords: boost, SABR
Rectal Dose Reduction Using Hydrogel in High- Dose-Rate Prostate Brachytherapy: Dosimetric Analysis Hathal Haddad 1 , Horst Hermani 2 , Michael Pinkawa 2 1 Department of Radiation Oncology, University Hospital Tübingen and Medical Faculty, Tübingen, Germany. 2 Department of Radiation Oncology, Wegeklinik, Bonn, Germany Purpose/Objective: High-dose-rate brachytherapy (HDR-BT) for prostate cancer provides excellent target conformity but may expose the rectum to significant radiation, particularly in combined-modality treatments. The injection of a hydrogel spacer between the prostate and anterior rectal wall aims to reduce rectal dose and subsequent toxicity. This study evaluated the dosimetric impact of hydrogel spacer placement on rectal dose parameters in HDR-BT compared with patients treated without hydrogel. Material/Methods: A total of 105 patients (232 HDR-BT fractions) were analyzed. Of these, 151 fractions were delivered with a prescription dose of 9 Gy (2 fractions of HDR-BT boost following EBRT) and 81 fractions with 11.5 Gy (HDR-BT monotherapy in three fractions). Before HDR-BT procedure, 10 mL hydrogel was injected transrectally under ultrasound guidance to separate the prostate from the anterior rectal wall. Dosimetric parameters were compared with a reference cohort treated with HDR-BT without hydrogel under identical planning conditions. The primary endpoints were the maximum rectal doses to 2 cm ³ (D ₂ cm ³ ) and 0.1 cm ³ (D ₀ . ₁ cm ³ ) of rectal wall, expressed as a percentage of the prescription dose. Results: Patient Characteristics:The hydrogel group comprised 105 patients (72 ± 8 years), while the non-hydrogel group included 102 patients (69 ± 9 years).Risk distribution (hydrogel group): low-risk = 10 (10%), intermediate-risk = 53 (50%), high-risk = 42 (40%).Risk distribution (without hydrogel): low-risk = 55 (54%), intermediate-risk = 47 (46%), high-risk = 0 (0%).The mean prostate volume was 44 ± 17 cm ³ in the hydrogel group and 41 ± 12 cm ³ in the non-hydrogel group. Patients treated with hydrogel were more frequently candidates for combined EBRT + HDR-BT due to higher risk profiles.Dosimetric Characteristics:Hydrogel spacer placement resulted in a substantial reduction in rectal dose. The mean rectum D ₂ cm ³ was 53 ± 9 % of the prescription dose with hydrogel versus 69–88 % without hydrogel (p < 0.01). The mean rectum D ₀ . ₁ cm ³ was 68 ± 11 % versus 116–163 % (p < 0.01), corresponding to an approximate 30 % reduction in D ₂ cm ³ and 70 %
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