Cortical versus medullary thymomas: a useful morphologic distinction?

We have tested the hypothesis that thymomas can be classified solely on the basis of epithelial cell morphology and that this distinction is prognostically useful. One hundred thymic epithelial tumors were classified according to the morphologic resemblance to cortical or medullary thymic epithelium and to the traditional classification (lymphocytic, epithelial, mixed, and spindled). Follow-up data was obtained on 78 patients. Fifty-eight percent of the tumors were invasive. Nineteen of the invasive tumors relapsed and none of the non-invasive tumors relapsed (P less than .0001). Four of nine tumors with microscopic invasion through the capsule recurred. Statistical analysis showed no significant differences in relapse-free survival for any of the histologic categories. Ninety-four percent of the tumors studied were keratin positive and all were chromogranin negative. Carcinoembryonic antigen was negative for all but one cytologically malignant tumor; of the tumors 75% were epithelial membrane antigen positive, 80% were Leu-7 positive, and 11% were neuron specific enolase positive. Seven of 12 tumors tested expressed HLA-DR. There was no correlation between immunoreactivity and classification. The morphologic cortical/medullary distinction is conceptually attractive but appears clinically to be no more advantageous than the traditional classification.