1. To compare the effects of the two types of thoracic esophageal cancer lymphadenectomy on the staging and prognosis of resectable esophageal cancer, which defined by the International Association of esophageal disease(ISDE) - standard mediastinal lymphadenectomy,total mediastinal lymphadenectomy and three field lymphadenectomy,and to find out reasonable range of lymphadenectomy.