Answer: Thymoma

Histology: This is a lymphocyte rich lesion which has a variety of morphologic appearances. In many areas, the lesion is dominated by small, inactive appearing lymphocytes. In other areas, a bland spindle cell component is more evident. These form short fascicles. Elsewhere, flattened epithelial cells form microcysts. By immunohistochemistry, the spindle and epithelial cells label for cytokeratin, while the lymphocytes label for T-cell markers such as CD3 and CD5.

The lymphocytes also label for CD99, indicating that they are of thymic origin.

Discussion: A solitary fibrous tumor would contain similar bland spindle cells. However, this lesion should label for CD34 and not cytokeratin, and would lack the dense lymphoid component of the current lesion. Lymphoblastic lymphoma would lack the spindle cell component of the current lesion, and lymphocytes would have a more primitive, “blastic”, appearance with mitotic activity. A synovial sarcoma would label less diffusely for cytokeratin, and would lack the admixed lymphoid component of the current lesion.

One pitfall of the current lesion is that the spindled cells of thymomas may be immunoreactive for CD20, typically thought of as a B-cell marker. Encapsulated spindle cell thymomas generally behave in an indolent fashion.

Reference(s):
– Am J Surg Pathol 2001;25:111-120.