S970
Clinical - Oligometastatic cancer
ESTRO 2026
Purpose/Objective: The role of stereotactic body radiotherapy (SBRT) in metastatic hormone-sensitiveprostate cancer (mHSPC) remains to be clearly defined. Phase II studieshave suggested that ablative treatment of all metastatic sites may actsynergistically with androgen receptor pathway inhibitor (ARPI) to enhance diseasecontrol. The START-MET trial aims to determine whether the addition of SBRT tothe standard of care (SOC) improves radiological progression-free survival(rPFS) in patients with oligometastatic mHSPC. Material/Methods: START-MET (ClinicalTrials.gov Identifier: NCT05209243) is a multicenter, randomized,open-label, phase III trial conducted by IRAD-FEOR. Eligible patientshad histologically confirmed oligometastatic hormone- sensitive prostate cancer ( ≤ 3lesions on CT/bone scan and ≤ 5 on PSMA or choline PET/CT) and no visceral metastases.Patients were randomized 1:1 to receive either SOC, comprising androgendeprivation therapy (ADT) plus an ARPI (abiraterone, apalutamide or enzalutamide ) andradiotherapy to the primary tumor if not previously treated or SOC plus SBRT to allmetastatic lesions.Patients were stratified according to PET/CT PSMA vs choline, and prior local treatmentto the primary tumor. The primary endpoint was rPFS (RECIST 1.1). Key secondaryendpoints include overall survival, time to PSA progression, time to chemotherapy, timeto castration resistance, quality of life, and safety. Results: Between February 2023 and September 2025, 230 patients of the 266 planned wereenrolled across 41 Spanish centers, and 216 were randomized (111 SOC+SBRT; 105SOC). The trial is still recruiting at this moment. Baseline and treatmentcharacteristics for the recruited patients were: median age 71 years; ECOG PS 0/1;92%/8%; predominant metastatic sites were bone (52%) and lymph nodes(33%); baseline median PSA was 2.9 ng/mL; prior local treatment was reported in65% of patients; SBRT was delivered to a median of 2,5 lesions. Treatment was welltolerated, with SBRT- related grade ≥ 2 acute toxicity observed in 3% of patients. Themost common adverse events were fatigue (28%), hot flushes (8%), and mild urinarysymptoms (6%). PSA levels decreased significantly in both groups, reaching a medianof 0,1 ng/mL at treatment completion. No safety issues were observed. Conclusion: This interim analysis confirms the feasibility of accrual and the favorable safetyprofile of combining SBRT with SOC in oligometastatic mHSPC. Updated efficacyresults, including rPFS, will be reported. START- MET represents the largestrandomized trial to date evaluating the role of SBRT in this clinical setting. Keywords: Prostate SBRT Oligometastases
Digital Poster 1983
Stereotactic Body Radiotherapy (SBRT) for Rib Metastases: Local Control and Toxicity Outcomes From a Contemporary Institutional Cohort Darren MC Poon 1 , Oi Lei Wong 2 , Hiu Yi Wong 2 , Sin Ting Chiu 3 , Bin Yang 4 , JIng Yuan 2 1 Comprehensive Oncology Center, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 2 Research Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 3 Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 4 Medical Physics Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong Purpose/Objective: Stereotactic body radiotherapy (SBRT) is widely used for osseous metastases; however, dedicated clinical data for rib lesions remain limited. Rib metastases pose unique challenges related to respiratory motion, proximity to lung parenchyma, and the risk of post- radiation fracture. This study evaluated the local efficacy and toxicity of SBRT for oligometastatic rib metastases treated within a contemporary clinical cohort. Material/Methods: A retrospective review identified 30 patients treated with SBRT for 39 rib metastases between 2020 and 2025. All lesions demonstrated PET avidity with corresponding CT changes. Target volumes were delineated on 4D-CT with PET fusion using 3–5 mm planning margins. SBRT was delivered via volumetric modulated arc therapy or helical TomoTherapy®, with or without active breathing control (ABC). Local failure was defined radiographically. Progression-free survival (PFS) included local, distant, or biochemical progression. Acute and late toxicities were clinician- reported. Results: Median follow-up was 11.9 months (range, 0.9–56.1). Most cases originated from prostate cancer (n=27). The predominant regimen was 35–40 Gy in 5 fractions (77%), corresponding to a median BED10 of 59.5 Gy. No local failures were observed. The 3-year overall survival was 84.6% (95% CI, 43.1–96.7%). The 3-year PFS was 39.6% (95% CI, 15.2–63.4%), with a median time to recurrence of 8.2 months. Acute toxicity was uncommon and mild, mainly Grade 1 fatigue, oesophagitis, and chest wall discomfort. Late toxicity included pulmonary fibrosis (n=1) and Grade 2 pneumonitis (n=1). No rib fractures occurred.Table 1. Patient and treatment characteristics. ABC, active breathing control; CBCT, cone-beam computed tomography; ECOG, Eastern Cooperative Oncology Group; SBRT, stereotactic body radiotherapy. *The biologically equivalent dose (BED) was calculated by
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