In 1961, Bernatz classified thymomas into four types according to their cellular composition: (1) epithelial cell type: epithelial cells occupy more than 2/3 of the tumor; (2) lymphocytic type
(2) lymphocytic type: lymphocytes account for more than 2/3; (3) mixed cell type: both of the above cell components reach more than 2/3; (4) spindle cell type: a large number of spindle cells occupy the interior of the tumor and are arranged in a bundle or spiral shape. Since this typing is of little significance in determining patient prognosis, Marino in 1985 reclassified thymic epithelial cells into three types according to their morphology: (1) cortical type: epithelial cells with round or ovoid nuclei, clear staining, centered nucleoli, and vesicular chromatin; (2) mixed type: thymoma epithelial cells mainly consist of medullary epithelial cells with cortical epithelial cell components; (3 ) medullary type: the epithelial cells are shuttle-shaped with long nuclei, the nucleoli are not easily visible, and the chromatin is diffuse. In 1999, WHO unified the typing of thymoma on the basis of the previous one: (1) Type A: that is, medullary or spindle cell thymoma. (2) Type AB: i.e., mixed type. (3) Type B: divided into 3 subtypes; Type B1: i.e. lymphocyte-rich thymoma, lymphocytic thymoma, cortical-dominant thymoma, or organoid thymoma; Type B2: i.e. cortical thymoma; Type B3: i.e. epithelial, atypical, squamous epithelioid thymoma, or well-differentiated thymoma. (4) Type C: i.e. thymic carcinoma, histologically this type has more malignant features than other types of thymoma. type A and AB are benign, type B and C are malignant. Clinical staging, in 1981, Masaoka observed that the epithelial cell type thymoma has a greater likelihood of metastasis and recurrence and a poorer prognosis, while the spindle cell type has a better prognosis. He also divided the tumor into four stages based on the growth observed during surgery, which is useful for judging the prognosis: stage I, with intact pericardium and no microscopic involvement of the pericardium, is called non-invasive thymoma; stage II, invading mediastinal adipose tissue or pleura, or microscopically seeing involvement of the pericardium, this stage is also non-invasive thymoma; stage III, involving important tissues such as pericardium, large blood vessels or lung. This stage is called invasive thymoma; stage IV, when pericardial or pleural implantation or distant metastasis occurs, also belongs to invasive thymoma. Non-invasive thymoma can be removed by surgery alone, but invasive thymoma requires supplemental radiotherapy after surgery to prevent recurrence and distant metastasis. The 5-year and 10-year survival rates for patients with stage III non-invasive thymoma are 85%-95% and 67%-80%, respectively, while the 5-year and 10-year survival rates for stage IIIIV invasive thymoma are only 44% and 27%, respectively.