Classification of Thymic Epithelial Neoplasms
Thymomas are tumors of thymic epithelial cells, with a variable admixture of lymphocytes. Traditional classifications have stratified thymomas by the relative proportions of lymphocytes and epithelial cells, with or without taking into account the shape of the epithelial cells (polygonal or spindle cell) (Table 1) (1–6). There are several difficulties with this approach. First, it attempts to be quantitative, yet it is in fact subjective. Second, the proportion of lymphocytes and epithelial cells often varies from one area to another in the same tumor. Finally, although some studies using these methods have shown that “epithelial-rich” tumors are more aggressive; all observers have agreed that only stage truly predicts outcome for patients with thymoma (Table 2) (7, 8, 5). Based on this experience, Levine and Rosai (4) proposed that thymomas not be classified at all based on histologic criteria, but simply stratified according to invasiveness as benign (noninvasive) and malignant (invasive). The rare cytologically malignant neoplasms of thymic origin were designated, thymic carcinomas (Table 3) (9–12).
KeywordsThymic Carcinoma Cytologic Atypia Thymic Epithelial Cell Thymic Epithelial Tumor Thymic Origin
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