Rectal Squamous Cell Carcinoma

The goal of this clinical trial is to learn if PD-1 monoclonal antibody combined with radical chemoradiotherapy works to treat rectal squamous cell carcinoma (rSCC). lt will also learn about the safety of the regime. The main questions it aims to answer are: Does PD-1 monoclonal antibody combined with radical chemoradiotherapy improve survival prognosis? What is the complete response rate (CCR) of the regime？ Researchers will compare PD-1 monoclonal antibody combined with radical chemoradiotherapy to previous study to see if this regime works to treat rSCCs. Participants will receive chemotherapy with DDP and 5-FU, immunotherapy with PD-1 monoclonal antibody and radiotherapy with a total dose of 50-54GY.