Type B3 Thymoma : Differential

Comments:
Type B3 thymoma vs Type B2 thymoma: Type B3 thymomas are epithelial-rich and lymphocyte-poor and appear pink at low magnification. Type B2 thymomas are lymphocyte-rich with only small clusters of epithelial cells and appear basophilic at low power. However, it must be remembered that there is a morphologic continuum from B2 to B3 thymomas and the distinction between the two may seem arbitrary in some cases. In addition, tumors with type B3 areas (epithelial-predominant) intermingled with bonafide type B2 component (more numerous lymphocytes than B3) are common.

Type B3 thymoma vs Squamous Cell Carcinoma (SCC): SCC lacks lobular growth and has infiltrative rather than pushing margins. Other features of SCC include prominent desmoplasia, lack of perivascular spaces, greater degree of cytologic atypia, and the presence of intercellular bridges. CD5 and CD117 are positive in >80% of thymic SCC, whereas they are generally negative in type B3 thymomas. TdT+ immature T-cells are absent in SCC but present in type B3 thymomas.

The image shows squamous whorls as well as abortive Hassall corpuscles that are commonly found in type B3 thymomas. Several squamous whorls can be seen in the center of this image.