Sintilimab Versus Mitomycin in Combination With Capecitabine and IMRT for Limited-stage Anal Squamous Cell Carcinoma

This is a prospective, multicenter, open-label, phase III randomized controlled clinical trial designed to evaluate the efficacy and safety of replacing the traditional chemotherapeutic drug mitomycin with the PD-1 inhibitor sintilimab in definitive chemoradiotherapy for limited-stage anal squamous cell carcinoma. The study plans to enroll 350 previously untreated patients with limited-stage anal squamous cell carcinoma and randomize them in a 1:1 ratio into two groups: the control group will receive the current standard treatment, namely intensity-modulated radiotherapy (IMRT) concurrent with capecitabine and mitomycin; the experimental group will receive an innovative "immunotherapy replacement" regimen, namely IMRT of the same technique concurrent with capecitabine and sintilimab. The study adopts a dual primary endpoint design, aiming to verify that the experimental group is non-inferior to the control group in the clinical complete response rate at 6 months after radiotherapy, and is significantly superior to the control group in the incidence of grade 3 or higher treatment-related acute toxicities.

Inclusion Criteria: 1. Histopathologically confirmed primary anal squamous cell carcinoma, including a subset of rectal squamous cell carcinomas. Definition for inclusion of rectal squamous cell carcinomas: According to Williams' criteria (1979) and WHO classification (2019), rectal squamous cell carcinomas included in this study must meet all of the following criteria: (1) Histopathologically confirmed as pure squamous cell carcinoma, excluding adenosquamous carcinoma; (2) Digital examination/endoscopy/MRI confirmation that the tumor mass is entirely located in the rectum (above the dentate line), with no evidence of primary origin in the anal canal or upward spread. If the inferior border of the tumor is within 5-6 cm from the anal verge, and the patient is scheduled to undergo radical radiotherapy and chemotherapy, the rectal squamous cell carcinoma can be diagnosed and screened for inclusion (When the primary epicenter cannot be determined, regardless of which side the tumor mass is biased towards, the diagnosis shall be considered for screening and inclusion as anal squamous cell carcinoma); (3) Female patients must be excluded for rectal metastasis from cervical squamous cell carcinoma (baseline gynecological examination is recommended). 2. Staged as cT2-T4N0M0 or cTanyN+M0 by imaging (AJCC 9th). 3. No prior tumor resection surgery (other than biopsy) or chemotherapy or other anti-tumor therapy. 4. Aged 18-75 years old. 5. ECOG performance status of 0-1. 6. Adequate organ function reserve: white blood cell (WBC) count ≥3 × 10\^9/L, absolute neutrophil count (ANC) ≥1.5 × 10\^9/L, hemoglobin (Hb) level \>90 g/L, platelet (PLT) count \>100 × 10\^9/L; serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels \