<qa>  What is the main cause of the patients death?Esophageal carcinoma is a common digestive system tumor, with number of deaths ranking No. 4 in malignant tumors. Esophageal squamous cell carcinoma in stage II, III is the most common in China. The esophageal cancer during these stages is no longer localized lesions, and cancer will be transferred to other organs of the body through blood transmission, leading to tumor metastasis, which is the main cause of the patients death. The tumor cells invade the surrounding tissues of primary tumor cells and enter the blood and lymphatic system, thus the circulating tumor cells CTC is formed and transported to a distal tissue, then effusing, adapting to the new microenvironment and finally forming metastases through the process of seeding, proliferation and colonization. 