Definition: Type A thymoma (also known as Spindle Cell Thymoma) is composed of bland spindle or oval epithelial cells admixed with few or no lymphocytes. Type A thymomas generally have a thick fibrous capsule as shown here. Prominent fibrous bands course through the tumor separating it into lobules. Capsular Invasion: Most type A thymomas (about 60%) are diagnosed at Stage I (completely encapsulated) according to the Masaoka-Koga classification system. About 30% of tumors are Stage II (microscopic capsular invasion OR gross invasion into surrounding fat OR adhesions to pleura or pericardium). The case shown here is Stage II. Note the tumor on both sides of the thick fibrous capsule.