Type AB thymomas may present with myasthenia gravis (18% cases), symptoms related to compression of anterior mediastinal structures (dyspnea, persistent cough, chest pain, fever, night sweats, and weight loss), or be asymptomatic and discovered incidentally. About two-thirds of cases are diagnosed at Stage I (completely encapsulated grossly and microscopically). Another 25% of cases are Stage II (microscopic invasion of capsule OR gross invasion into surrounding mediastinal fat OR adhesions to pleura or pericardium, but no gross invasion of these structures). In type AB thymomas, the lymphocyte-poor A component and the lymphocyte-rich B component may be sharply demarcated (as seen here) or be intimately intermingled. The right half of this image shows type A areas composed of spindle cells and displaying microcystic growth pattern. The left half shows a sharply-defined nodule containing abundant lymphocytes admixed with spindle epithelial cells.