Type A thymoma is an organotypic thymic epithelial neoplasm composed of bland spindle/oval epithelial tumor cells with few or no lymphocytes. Alternate designations include spindle cell or medullary thymoma (see Chapter 99). The tumor has been postulated to derive from the normal thymic medullary epithelial cells.¹

Type A thymoma is a relatively uncommon type of thymoma and corresponds to 4% to 19% of all thymomas.^1,2 No consistent gender predilection has been reported. The age at manifestation ranges from 32 to 83 years, with a mean age of 61 years.¹ Type A thymoma may have an associated autoimmune disorder, including but not limited to myasthenia gravis (4% to 24%), Good syndrome, and pure red cell aplasia.^3,4 Patients with type A thymoma may experience symptoms caused by compression of the surrounding organs by an expansive mass. These problems may take the form of superior vena cava syndrome, dysphagia, cough, or chest pain. Some patients with type A thymoma have no symptoms at all, and the tumor can be detected for an unrelated problem as an enlarged mediastinal area or mass by x-ray, CT, or MRI.

Grossly, type A thymoma is usually well circumscribed and encapsulated (Fig. 99.1). The cut surface is tan white and shows vague lobulation with less distinct dissecting white fibrous bands than is seen in other types. Cystic change or calcification of the capsule may be seen. Type A thymomas, just like any other histologic variant of thymoma, have a similar potential to become invasive tumors (capsular invasion) capable of spreading locally or outside of the thoracic cavity.⁵

Histologically, the main morphologic growth pattern is that of a spindle cellular proliferation arranged in short fascicles of spindle- and/or ovalshaped cells with elongated nuclei, dispersed chromatin, and inconspicuous nucleoli (Figs. 99.2 and 99.3). The tumor cells can form a variety of histologic structures.¹ They are arranged in solid sheets without any particular pattern or in a storiform pattern. The tumor cells can form cysts of various sizes, glandular structures, glomeruloid bodies, rosettes with or without a central lumen, hemangioma-like papillary projections in cystic spaces, or meningioma-like whorls. Thin-walled branching vessels in the background may impart a hemangiopericytoma-like appearance. Mitoses are seldom found, but lobular infarcts can occur. Type A thymomas often have few or no lymphocytes and show neither distinct lobules nor dissecting fibrous bands or prominent perivascular spaces as seen in other types of thymoma. Occasional cases may display atypical features (mild to moderate atypia, increased mitotic figures, focal necrosis, “atypical type A thymoma”) somewhat resembling B3 thymoma, except for the fact that the tumor cells are spindled instead of round/polygonal and lack intratumoral cortical immature T lymphocytes.⁶

The immunoprofile of type A thymoma remains one of the most controversial questions in the literature. Different studies have generated conflicting results related to antigen expression due to differences in
detection systems and antibodies used. The tumor cells, similar to normal medullary thymocytes, are strongly positive for EMA (epithelial membrane antigen) (Fig. 99.4), CK8/18, and vimentin. Other cytokeratins of different molecular weights and p63 show variable expression.⁷ CD20-positive tumor cells may be detected focally.⁸ There is no expression of CD5, and BCL-2, CD57, and CK5/6 are variable and focal.^9,10 Basement membrane-like deposits as demonstrated by antilaminin and anti-type IV collagen antibodies surround most tumor cells. P53 protein and Ki 67 show only low or no expression. The few intratumoral lymphocytes express mature T-cell markers, CD3 and CD5 (Fig. 99.5). Cortical immature T cells (TdT, CD1a, and CD99+), seen in B1, B2, and B3 thymomas, might be absent in type A (Fig. 99.6).⁸