Thymoma is a tumor derived from thymic epithelial cells. Unlike other tumors, the benign and malignant nature of thymoma cannot be determined on the basis of histology, and its benignity is based on the presence of perineural infiltration, surrounding organ invasion, or distant metastasis. Some scholars use the term “benign thymoma” to describe a non-invasive thymoma. In fact, recurrence and metastasis can occur after surgical resection of thymoma of any tissue type, including stage I. Therefore, all thymomas are currently considered potentially malignant, and the term “benign thymoma” should be discarded. In order to avoid mentioning “malignant”, it is currently advocated to classify thymomas as non-invasive or invasive. In general, 30% to 40% of thymomas are invasive. The traditional histological classification is based on the ratio of non-neoplastic lymphocytes to tumor epithelial cells in the tumor and is divided into lymphocytic, mixed cell, and epithelial cell types. this classification has been used for many years, but since it cannot determine the benignity or malignancy of the tumor, let alone the prognosis, and most patients are of mixed type, it has little clinical guidance. in 1985 Muler. Hermelink et al. proposed a new classification method: histologically and immunophenotypically based typing, similar to the cortical or medullary regions of the thymus, into medullary, mixed, cortical-dominant and cortical types, and this classification method was widely used. In order to better relate the histological typing of thymoma to the aggressiveness and prognosis of the disease, WHO developed an updated classification of thymic epithelial tumors in 1999, which adopts the Muller-Hermelink classification and classifies thymoma into 3 types, A, B and AB, based on the morphology of epithelial cells and the ratio of lymphocytes to epithelial cells, with type A tumors consisting of spindle-shaped Type A tumors consist of spindle-shaped tumor epithelial cells without atypical or tumor lymphocytes; type B tumors consist of round epithelial-like cells; type AB is a mixture of the two, similar to type A but containing tumor lymphocytes. Based on the proportional increase of epithelial cells and the appearance of atypical tumor cells, type B tumors are further divided into three subtypes: type B1, type B2, and type B3. All thymic carcinomas are type C. Table Relationship between WHO classification of thymoma and Muller-Hermelink classification WHO classification Muller-ttermelink classification A-type thymoma Medullary thymoma AB-type thymoma Mixed thymoma B1-type thymoma Cortical-dominant thymoma B2-type thymoma Cortical thymoma B3-type thymoma Well-differentiated thymic carcinoma c type thymoma Malignant thymic carcinoma 1.5% of thymomas are type A, 1/3 are type AB, 10%-20% are type B1, 20%-35% are type B2, and 10%-15% are type B3. To date, several studies have evaluated the significance of the WHO classification system. okumura et al. summarized 273 patients with thymoma, the proportion of aggressive thymoma in each type A, AB, B1, B2, and B3 was 11.1%, 41.6%, 47.3%, 69.1%, and 84.6%, respectively. 20-year survival rates were compared, and tumors of types B2 and B3 were more aggressive and patients The survival period was shorter.