Microscopic description and IHC:

Sections of the biopsy show lymphoepithelial tissue with effaced thymic architecture composed of small lymphocytes with clumpy chromatin. Fibrous septa are also present.

AE1/3, CK5/6 (not shown), and P63 highlight “wall to wall” network of epithelial cell processes as well as immature thymic lymphocytes (CD1a and TdT positive).

Discussion:

Thymoma are low to intermediate grade epithelial lesions arising in the thymus and are commonly located in the anterosuperior mediastinum. They can also be found in other adjacent anatomic regions. Together with thymic carcinoma, they are the most common neoplasms in the thymus in adults.

Slightly less than half of the thymoma patients also have myasthenia gravis. Other paraneoplastic syndromes can also exist.

Grossly, thymoma are encapsulated and have minimal capsular invasion. The lymphocytes are T cells, lack the blastic chromatin of lymphoblasts, and stain for TdT, CD1a, CD3. The epithelial cells are reactive for p63, CK5, and cytokeratin. Thymic carcinoma can have a more poorly differentiated large cell carcinoma like appearance and can have geographic necrosis. They are positive for CD5 and CD117 (c-kit). Grossly, they are not encapsulated and have no internal fibrous septation.

The classification of thymoma is based on the presence of lymphocytes, epithelial cells, or mixed tumor cell population, according the World Health Organization (WHO) system.

The differential diagnosis for commonly found anterior mediastinal masses are often referred to as the three terrible Ts: thymoma, teratoma/germ cell tumor, (terrible) lymphoma, and thyroid tissue.

Table depicting Thymosa
References:

Pathology & genetics: Tumours of the lung, pleura, thymus and heart. In: World Health Organization Classification of tumours, Travis WD, Brambilla E, Muller-Hermelink HK, Harris CC (Eds), IARC Press, Lyon, France 2004.

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