Evaluation of the malignant grade of thymoma by morphometric analysis.

To determine the histologic grade of malignancy of thymoma, the nuclear areas of epithelial cells of 42 thymomas, 14 thymic hyperplasias, and ten normal thymuses were measured by morphometry and the results were compared with the extent of disease, association with myasthenia gravis (MG), and histologic subtypes of the thymomas. The nuclei of epithelial cells of thymomas were significantly larger than those of both of thymic hyperplasias and normal thymuses (P less than 0.01). The nuclei of epithelial cells of invasive thymomas were significantly larger than those of noninvasive ones (P less than 0.001). In the noninvasive group, the nuclei of epithelial cells of thymomas associated with MG were significantly larger than those without MG (P less than 0.02), and were similar to those of invasive thymomas. These results confirm that invasive thymomas are morphometrically malignant tumors, and suggest that noninvasive thymomas associated with MG might have malignant potential.