Overview: This Phase 2 randomized trial compares adriamycin and ifosfamide chemotherapy combined with sintilimab, a PD-1 checkpoint inhibitor, against chemotherapy alone as second-line treatment for patients with advanced or unresectable soft tissue sarcoma. The study aims to determine whether adding sintilimab to standard chemotherapy can improve outcomes for patients whose disease has progressed or who cannot tolerate first-line therapy.