Malignancy and differentiation of neoplastic epithelial cells of thymoma

Fifteen fetal thymuses and 100 thymomas were examined concerning the degree of differentiation of thymic epithelial cells by light and electron microscopy. The following results were obtained. The embryologic process of the fetal thymus was categorized into four stages (8, 12, 15, and 17 weeks or more of gestation) based on the morphological features of epithelial cells and the maturity of thymic tissues. Thymomas were classified into the undifferentiated, and poorly, moderately, and well-differentiated types by their morphological features in comparison with the fetal thymuses. The incidences of the respective types in the 100 thymomas were 5, 16, 68, and 11 cases. The moderately differentiated type was divided into three subtypes; namely, polygonal cell (49 cases), clear cell (6), and spindle cell type (13). The degree of differentiation of the thymomatous epithelial cells showed strong correlations with both the invasiveness and the prognosis of the thymoma. The ratios of invasive thymomas were 36, 47, 25, and 100% for the well-differentiated, moderately-differentiated, poorly-differentiated, and undifferentiated types, respectively. The 10-year survival rates were 87.5, 62.5, 46.9 and 0% for the well-differentiated, moderately-differentiated, poorly-differentiated, and undifferentiated thymomas.