S1185
Clinical - Urology
ESTRO 2026
1 Medical Physics, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy. 2 School of Medicine and Surgery, University of Milan Bicocca, Milan, Italy. 3 Radiation Oncology, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy Purpose/Objective: SBRT is increasingly used for localized prostate cancer (PCa), yet optimal patient selection remains debated. In particular, the influence of prostate volume and baseline urinary symptoms on treatment toxicity is not fully established. This study investigated their role in predicting acute GU and GI side effects across three SBRT regimens at a single institution. Material/Methods: We retrospectively analyzed 251 patients with localized PCa treated with SBRT: 30 Gy/3 fractions (n=46), 36.25 Gy/5 fractions (n=99), and 42.7Gy /7 fractions (n=106). Baseline prostate volume and International Prostate Symptom Score (IPSS) were collected. Acute toxicity was graded according to CTCAE v5, with G2+ events considered clinically relevant. Multivariable logistic regression models were fitted, adjusting for fractionation, ADT, treatment duration, and comorbidities. Receiver operating characteristic (ROC) curve analyses were conducted to determine clinically meaningful thresholds for risk stratification. Results: Overall rates of acute GU and GI G2+ side effects were 32.3% and 9.6%, respectively, with no major differences across fractionation regimens. Larger prostate volumes were strongly associated with acute GU symptoms (OR 1.03 per cc, p<0.001), independent of fractionation. Patients with GU events had a median prostate volume of 68 cc versus 55 cc in those without (p<0.001). A threshold of ~90 cc identified patients at higher risk. Higher IPSS predicted GU toxicity (OR 1.16 per point, p=0.019), but the effect varied by fractionation. In the 3-fraction group, patients with IPSS ≥ 13 had a markedly higher GU toxicity rate (52.9% vs. 6.9%, p=0.001). This association was not observed in the 5- and 7-fraction groups. This was likely due to the higher accumulated dose over a shorter overall treatment duration (7 vs. 11 vs. 17 days, p<0.001). Prostate volume also influenced acute GI toxicity in the 3-fraction group (OR = 1.05 per cc, p=0.021), with risks increasing above 85–90 cc (AUC > 0.80), while no effect was seen with longer schedules. Conclusion: This large single-institution series shows that prostate volume and IPSS are strong predictors of acute toxicity after SBRT. Patients with prostate volume ≥ 90 cc were at higher risk regardless of the fractionation used. Poor baseline urinary function particularly impaired tolerance to ultra-hypofractionated regimens. More extended schedules (5–7 fractions) may offer safer
alternatives in these patients, providing practical selection criteria to improve patient safety and personalize prostate SBRT. Keywords: Volume, IPSS, SBRT
Digital Poster 1523
Single-Arc versus Dual-Arc SBRT for Localized Prostate Cancer: a propensity-weighted analysis Denis Panizza 1,2 , Valeria Faccenda 1,2 , Federica Ferrario 3 , Lorenzo De Sanctis 2 , Giulia Rossano 2 , Riccardo Ray Colciago 2 , Elena De Ponti 1,2 , Stefano Arcangeli 2,3 1 Medical Physics, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy. 2 School of Medicine and Surgery, University of Milan Bicocca, Milan, Italy. 3 Radiation Oncology, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy Purpose/Objective: Two Phase III trials (PACE-B and HYPO-RT-PC) have standardized the use of SBRT in localized prostate cancer (PCa), adopting either 36.25 Gy in 5 fractions or 42.7 Gy in 7 fractions. However, treatment planning often depends on available technology and vendor- specific solutions. In our standard linac-based SBRT workflow, treatments were initially delivered with two arcs; later, a one-arc approach was introduced to reduce treatment time, while further reducing rectal and bladder mean doses. This study aimed to investigate whether this evolution in treatment technique affected side effect outcomes. Material/Methods: We retrospectively analyzed all patients with localized PCa treated with SBRT at our center between 2022 and 2024. PTV margins were 5/3mm posteriorly in accordance with international guidelines. Acute genitourinary (GU) and gastrointestinal (GI) side effects, occurring within 6 months post-treatment, were graded using CTCAE v5. Multivariate logistic and Cox regression analyses (MVA) were first performed to identify clinical predictors of each outcome. To assess the independent impact of arc number and dose- volume parameters, propensity score matching (PSM) and weighted logistic regression were applied, both adjusted for the significant variables identified by MVA. Results: A total of 205 patients (n=65 one-arc, n=140 two-arcs) were included, with a median follow-up of 23 months. Acute GU G2+ occurred in 31%, and GI G2+ in 10%. Median treatment time per fraction decreased from 2.1 min (range, 1.7–3.0) to 1.5 min (range, 1.3–1.9) with one-arc planning. Larger prostate volume was significantly associated with acute GU G2+ (OR=1.03, p<0.001), and younger age with acute GI G2+
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