Hypofractionated Radiotherapy Combined With Immunochemotherapy for Conversion Treatment of Gastroesophageal Junction Adenocarcinoma

In Phase Ib, HFRT will be administered at one of three dose levels: 3 Gy × 5 fractions, 4 Gy × 5 fractions, or 5 Gy × 5 fractions. The recommended dose determined in Phase Ib will be used in Phase II (delivered as 5 fractions).

SOX chemotherapy regimen: Oxaliplatin 130 mg/m² administered by intravenous infusion on Day 1; S-1 administered orally for 14 consecutive days followed by a 7-day rest period. The dosage of S-1 is based on body surface area (BSA): 40 mg twice daily for BSA ≤1.5 m², 50 mg twice daily for BSA between 1.5-1.6 m², and 60 mg twice daily for BSA ≥1.6 m². Each cycle is repeated every 3 weeks.

Sintilimab 200 mg administered by intravenous infusion on Day 1 of each 3-week cycle.