[Carcinoma showing thymus-like differentiation of the thyroid: a study of 2 cases].

Objective: To describe the pathologic features and differential diagnosis of carcinoma showing thymus-like differentiation (CASTLE) of thyroid.

Methods: The clinical findings, morphologic features and immunohistochemistry (EnVision) of 2 cases of CASTLE were studied.

Results: Macroscopically, the tumor appeared as a hard grayish-white and slightly lobulated mass. Histologic examination revealed well-circumscribed islands of tumor cells associated with desmoplastic stroma. The tumor cells were polygonal to spindle in shape and contained lightly eosinophilic cytoplasm, oval nuclei and small distinct nucleoli. The nuclear atypia was mild to moderate and the mitotic count measured 1 to 2 per 10 high-power fields. Immunohistochemical study showed that the tumor cells expressed CD5 and CD117.

Conclusions: CASTLE is a rare type of thyroid carcinoma with distinctive morphologic findings. It needs to be distinguished from undifferentiated thyroid carcinoma, squamous cell thyroid carcinoma, metastatic lymphoepithelioma-like carcinoma and follicular dendritic cell sarcoma. Immunohistochemical staining for CD5 and CD117 is helpful in confirming the diagnosis.