S1157
Clinical - Urology
ESTRO 2026
Aranda 2 , Bruno Zambrana 2 , Rosa Alonso 2 , Carmen Rubio 2 1 Radiation Oncology, Universidad Militar Nueva Granada - Instituto Nacional de Cancerología, Bogotá, Colombia. 2 Radiation Oncology, HM Hospitales, Madrid, Spain. 3 Radiation Oncology, Institute of Oncology Ljubljana, Ljubljana, Slovenia. 4 Faculty of Medicine, Univesity of Ljubljana, Ljubljana, Slovenia Purpose/Objective: To evaluate the feasibility, safety, and tolerance of ultra-hypofractionated (UHF) pelvic nodal irradiation (PNI) with or without simultaneous integrated boost (SIB) in prostate cancer patients treated with stereotactic ablative radiotherapy (SABR) in the definitive or post-prostatectomy (adjuvant/salvage) setting. Material/Methods: Between September 2019 and October 2024, 114 men with prostate cancer received SABR including elective PNI (Table 1). Treatment consisted of volumetric modulated arc therapy (VMAT) delivering 36.25–40 Gy in 5 fractions of 7.25–8 Gy every other day to the prostate plus seminal vesicles or surgical bed (RTOG- based contours). PNI was prescribed to 26 Gy in 5 fractions of 5.2 Gy, with SIB up to 40 Gy in 5 fractions. In 25 patients (22%), PSMA- or choline-PET–positive lymph nodes received the same SIB dose.Gold fiducial markers were implanted for image guidance, and a 100–120 cm ³ air-filled endorectal balloon was used for rectal stabilization. Patients were instructed to empty the bladder and drink 500 mL of water 30 minutes before irradiation. Daily ExacTrac® and cone-beam CT ensured accurate positioning and intrafraction monitoring. All received an α -1 receptor antagonist from 7 days before to 1 month after SABR and 4 mg dexamethasone on treatment days. Androgen deprivation therapy was administered in 79 patients (median duration 24 months, range 6–36).Acute and late toxicities were graded per CTCAE v5.0 at 1 month, every 3 months for 2 years, and every 6 months thereafter.
Results: Both regimens were well tolerated, with no ≥ grade 3 acute or late toxicities. Only grade 2 acute GU toxicity was higher with UHF as compared to MHF (33.3% vs 3.8%, p = 0.007). Treatment related adverse effects are detailed in table 2In MHF cohort 16 patients (62%) remained progression-free and 10 (38%) relapsed; 4 (17%) developed new metastases (2 extrapelvic nodal, 2 vertebral, 1 iliac bone, 1 pulmonary). In the UHF group, overall and progression-free survival (PFS) were both 100%, with no relapses or deaths. Actuarial PFS at 12/24 months: 100%/79.6% (MHF) vs 100%/100% (UHF), p = 0.131. Conclusion: Both schedules were effective and safe. UHF offers a shorter, resource-efficient alternative with potential quality-of-life benefits, at the cost of a higher likelihood of mild-to-moderate acute genitourinary toxicity. Larger prospective studies with longer follow- up are warranted. Keywords: Oligometastases, de novo, SABR Digital Poster 907 Ultra-Hypofractionated Pelvic SABR for Prostate Cancer: Pushing the Boundaries of Nodal Irradiation Yuli Otero 1 , Angel Montero 2 , Helena Zobec Logar 3,4 , Ovidio Hernando 2 , Mercedes Lopez 2 , Jeannete Valero 2 , Beatriz Alvarez 2 , Emilio Sanchez 2 , Raquel Sanchez 2 , Raquel Ciervide 2 , Xin Chen-Zhao 2 , Mariola Garcia-
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