S1307
Clinical - Urology
ESTRO 2026
Purpose/Objective: Stereotactic body radiation therapy (SBRT) has emerged as a treatment option for localized intermediate-risk prostate cancer, providing high biological effectiveness and favourable toxicity. The planning study aimed to evaluate the clinical outcomes and treatment-related toxicity of SBRT in patients treated at our institution. Material/Methods: Between September 2020 and June 2025, 179 consecutive patients with intermediate-risk prostate cancer were treated with SBRT (36.25 Gy in 5 fractions) using the CyberKnife® system with hydrogel spacer placement. A total of 121 patients who had a follow-up period longer than 12 months after treatment were included in the analysis. Acute genitourinary (GU) and lower gastrointestinal (GI) toxicities and late morbidity were evaluated according to the Radiation Therapy Oncology Group (RTOG) toxicity scoring system. Results: The mean age at diagnosis was 74 years (range 53-84) and the mean prostate-specific antigen (PSA) value was 8.47 ng/mL (range 1.98-18.8), with 77.7% of patients having a PSA level < 10 ng/mL. The distribution of Gleason score (GS) at the time of diagnosis was as follows: GS 6 was observed in 5% of patients, GS 7 (3+4) in 38%, GS 7(4+3) in 56.2%, and GS 8 in 0.8%. Clinical staging data showed 47.9% of patients with cT1 disease, 51.3% with cT2 and 0.9% with cT3. Median follow-up was 29 months (range, 12– 56). Biochemical control was achieved in 95% of patients, and biochemical recurrence occurred in 5% (two cases with isolated biochemical relapse and three with clinical recurrence: one local relapse confirmed by PSMA-PET, one nodal, and one combined bone and nodal). Acute GU toxicity was grade 1 in 52.9% of patients, grade 2 in 2.5% and grade 3 in 0.8%. Acute GI toxicity was grade 1 in 1.7% and grade 2 in 0.8%. Late GU toxicity observed was grade 1 in 29.8% of patients, grade 2 in 0.8% and grade 3 in 1.7%. Late GI toxicity was limited to grade 1 in 0.8% of cases, with no ≥ grade 2 events. Conclusion: SBRT for intermediate-risk prostate cancer demonstrates excellent short-term biochemical and clinical control with minimal late lower gastrointestinal and genitourinary toxicity. Longer follow-up is needed to assess long-term efficacy and late toxicity outcomes. References: Wang K., Mavroidis P., Royce T.J., Falchook A.D., Collins S.P., Sapareto S., Sheets N.C., Fuller D.B., El Naqa I., Yorke E., et al. Prostate Stereotactic Body Radiation Therapy: An Overview of Toxicity and Dose Response. Int. J. Radiat. Oncol. Biol. Phys. 2021;110:237– 248Kishan A.U., Dang A., Katz A.J., Mantz C.A., Collins S.P., Aghdam N., Chu F.-I., Kaplan I.D., Appelbaum L.,
Fuller D.B., et al. Long-term Outcomes of Stereotactic Body Radiotherapy for Low-Risk and Intermediate-Risk Prostate Cancer. JAMA Netw. Open. 2019;2:e188006. Keywords: prostate cancer, SBRT, toxicity
Digital Poster 4847
Assessing the effectiveness of robust optimization in hypofractionated VMAT with SIB for prostate cancer to mitigate setup uncertainties Hela Ayouni 1,2 , Semia Zarraa 3 , Eya Azzabi 3 , Khedija Ben Zid 3 , Sarra Saidi 3 , Bilel Daoud 4 , Lotfi Ben Salem 2 , Chiraz Nasr 3 1 Medical Physics, Higher Institute of Medical Technologies of Tunis, Tunis, Tunisia. 2 Department of Medical Physics, Salah Azaiz Institute, Tunis, Tunisia. 3 Department of Radiation Oncology, Salah Azaiz Institute, Tunis, Tunisia. 4 Department of Imaging physics research, MD anderson cancer center, Texas, USA Purpose/Objective: Geometric uncertainties, particularly setup errors, can compromise the quality of prostate radiotherapy. Conventional PTV margins do not explicitly account for organs at risk (OARs), which can increase toxicity. While robust optimization has been investigated to mitigate these uncertainties, its use in hypofractionated VMAT with a simultaneous integrated boost (SIB) remains unexplored. This study therefore aims to evaluate the dosimetric efficacy of robust optimization in comparison with PTV-based planning, with an emphasis on enhancing treatment robustness and minimizing OAR exposure. Material/Methods: A total of ten patients diagnosed with prostate cancer were included in this retrospective study. Their treatment involved two dose levels, 68Gy and 50Gy, administered over 25 fractions. For each patient, two sets of VMAT plans were created: one based on the PTV and the other employing robust optimization. The plan robustness was assessed by generating perturbed dose plans that incorporated setup uncertainties resulting from isocenter shifts. The magnitude of these uncertainties was matched to the patient setup uncertainty used in robust optimization, which was identical to the PTV margin (7 mm isotropic for PTV50 and 7 mm isotropic for PTV68, except 5 mm posteriorly). The homogeneity index (HI), target coverage, and three organs at risk (OARs) doses were analyzed, and a t-test was used to assess differences between robust and PTV plans. Results: There was no statistically significant variation in the D95% of CTVs between the two approaches. However, the HI of CTV-68 showed marginal enhancement with
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