Immunotherapy Plus Anlotinib After Surgery for Esophageal Squamous Cell Cancer With Residual Tumor and Positive Lymph Nodes

The goal of this clinical trial is to learn whether postoperative immunotherapy combined with anlotinib can help prevent or delay cancer recurrence in adults aged 18 to 75 years with locally advanced esophageal squamous cell carcinoma. Eligible participants must have previously received chemotherapy combined with anti-PD-1 immunotherapy, followed by complete surgical removal of the cancer. Examination of the surgical specimens must show more than 10% viable tumor remaining in the original tumor bed and cancer cells remaining in regional lymph nodes. The main questions this trial aims to answer are: * What proportion of participants are alive without cancer recurrence or a second primary cancer 12 months after enrollment? * How long do participants remain free of cancer recurrence, and how long do they survive? * What treatment-related medical problems occur during treatment with anti-PD-1 immunotherapy plus anlotinib? * Where does the cancer recur if recurrence occurs? All participants will receive the same study treatment; there is no comparison group. Participants will: * Receive an anti-PD-1 immunotherapy medicine by intravenous infusion once every 3 weeks, generally using the same anti-PD-1 medicine received before surgery * Take anlotinib by mouth once daily for 14 days, followed by 7 days without anlotinib, in each 21-day treatment cycle * Continue treatment for up to 15 cycles unless the cancer returns, unacceptable side effects occur, consent is withdrawn, or another reason for stopping treatment arises * Undergo regular clinic visits, laboratory tests, physical examinations, and safety assessments during treatment * Undergo imaging examinations approximately every 3 months to check for cancer recurrence * Be followed for disease recurrence and survival for up to 5 years after surgery Stored surgical tumor tissue will also be studied to explore whether features of the tumor immune environment are associated with treatment outcomes.