Carcinoma with thymus-like differentiation arising in the dermis of the head and neck.

Carcinoma exhibiting thymus-like differentiation (CASTLE) is a rare, distinct tumor of the thyroid gland or soft tissue of the head and neck that may simulate primary squamous cell carcinoma or lymphoepithelioma, and which contains features reminiscent of thymic differentiation including Hassall's corpuscles, occasional perivascular spaces, and the presence of lymphocytes. Ectopic thymic tissue may result from incomplete descent or persistence of the cervical portion of the thymus and may occur anywhere along the course of the embryonic descent from the angle of the mandible to the sternal notch. Herein, we report two cases of dermal extrathyroidal CASTLE. The differential diagnosis of squamoid carcinoma with features of thymic differentiation includes extrathyroidal CASTLE, a primary squamous cell carcinoma with thymic differentiation, lymphoepithelioma-like carcinoma of the skin, and metastatic squamous cell carcinoma of unknown primary. It is essential that the latter two be ruled out before accepting the diagnosis of an extrathyroidal carcinoma with thymus-like differentiation.