Type A Thymoma : Microscopic
This Type A thymoma is composed of spindle shaped cells arranged in short fascicles. They have oval or round bland nuclei with finely dispersed chromatin and inconspicuous nucleoli. Mitotic activity is low. A few gland-like structures with eosinophilic secretions can be seen on the left. There are only rare immature thymocytes scattered among the spindle cells. If there are lymphoid aggregates OR greater than 10% of the tumor has moderate infiltrate of lymphocytes – such tumors are best classified as type AB thymomas. Rare cases of type A thymoma show cytologic atypia, increased mitotic activity, and focal necrosis. The prognostic significance of this finding is not known.