Malignant grading of cortical and medullary differentiated thymoma by morphometric analysis

Forty-seven thymomas were histologically classified into cortical, mixed, and medullary types, and the malignant grade of each subtype was evaluated by measuring the nuclear areas of epithelial cells. Fourteen normal thymuses and 23 thymic hyperplasias were used as controls. The cortical thymomas were more frequently invasive and associated with myasthenia gravis than the medullary type (P less than 0.01 and 0.05, respectively). Cortical thymomas had larger epithelial cell nuclei than the medullary type (P less than 0.001). Cortical and medullary thymomas had larger epithelial cell nuclei than the thymic cortex and medulla (P less than 0.001 and 0.01, respectively). The thymic cortex had larger epithelial cell nuclei than the medulla (P less than 0.001). These results confirm that the epithelial cell nuclei of cortical and medullary thymomas correspond morphometrically to the thymic cortex and medulla, respectively, and that most cortical thymomas have a higher malignant grade than the medullary type.