S896
Clinical - Mixed sites & palliation
ESTRO 2026
and efficacy of 5-fraction SBRT regimen for the management of spinal metastases. Keywords: Spine Metastases, Fractionation, Local Control
Mini-Oral 3359 Phase I Trial of Ablative 5-Fraction Stereotactic Magnetic Resonance-Guided Adaptive Reirradiation for Abdominal and Pelvic Tumors Michael D Chuong 1,2 , Kathryn E Mittauer 1,2 , Robert Herrera 1 , Adeel Kaiser 1,2 , Yongsook C Lee 1,2 , Tatiana Bejarano 1,2 , Matthew D Hall 1,2 , Seleshi Demissie 3 , Carolina Rojas 1 , Alonso N Gutierrez 1,2 , Carla Salazar Monteagudo 1 , Ranjini Tolakanahalli 1,2 , Minesh P Mehta 1,2 , Rupesh Kotecha 1,2 1 Radiation Oncology, Miami Cancer Institute, Miami, USA. 2 Oncological Sciences, Florida International University Herbert Wertheim College of Medicine, Miami, USA. 3 Office of Clinical Research, Miami Cancer Institute, Miami, USA Purpose/Objective: Reirradiation (reRT) is often limited to palliative doses resulting in suboptimal long-term efficacy. MR-guided linacs may facilitate ablative dose escalation for 5- fraction reRT in the abdomen and pelvis although prospective evidence is not available. Material/Methods: We conducted a fixed-dose phase I trial (NCT06397573) of reRT prescribed to 50 Gy in 5 fractions (BED = 100 Gy10) on a 0.35 Tesla MR-linac. Key eligibility criteria included tumor in the abdomen or pelvis not invading or originating from GI organs-at- risk (OARs), 1 prior RT course ≥ 6 months prior with prescribed EQD2 >40 Gy, and overlap of the prior and reRT 50% isodose lines. Continuous cine-MRI, automatic gating, and online adaptive radiotherapy (oART) were required. CTV use (2-10 mm expansion from GTV) was optional, prescribed to the same dose as GTV. ReRT GI OAR constraints were V33Gy<0.5 cc and V35Gy<0.03 cc. The primary endpoints were acute grade 3+ and any grade 5 adverse events (AEs). Secondary endpoints were 1-year freedom from local progression (FFLP), 1-year overall survival (OS), late grade 3+ AEs, and quality of life (QOL) using the FACT- G survey. A sample size of 10 patients was determined pragmatically. Results: All patients completed reRT in consecutive days for locoregional (80%) or metastatic (20%) recurrence located in the pancreas (50%) or lymph nodes (50%). Primary sites included pancreas (60%), colon/rectum (20%), lung (10%), and endometrium
Conclusion: Five-fraction spine SBRT provided durable LC and favorable survival outcomes with minimal toxicity. Dose escalation improved LPFS in patients with breast cancer. Younger age, prostate or breast primary, and absence of visceral metastases were associated with improved prognosis. These results support the safety
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