OBJECTIVETo study the clinicopathologic features, immunophenotypes and differential diagnosis of thyroid carcinoma showing thymus-like differentiation (CASTLE).

METHODSThe clinical and pathologic features of 8 cases of CASTLE were reviewed. Immunohistochemical study was performed using a panel of antibodies. In-situ hybridization for Epstein-Barr virus-encoded RNA (EBER) was also carried out.

RESULTSThere were altogether 4 males and 4 females. The age of the patients ranged from 25 to 57 years (mean = 48.8 years). All of them presented with painless mass at the anterior neck. Two patients also complained of hoarseness of voice. On CT scan, the tumor had a low density with contrast enhancement. Seven cases were located in the mid to lower pole and the remaining one in the upper pole of thyroid gland. Four cases were relatively circumscribed. The other 4 cases showed evidence of extrathyroidal invasion. Grossly, the tumor had a nodular or lobulated appearance and was gray-white in color, with a mean diameter of 4.3 cm. Microscopically, the tumor was infiltrative and consisted of islands, nests or lobules of epithelial cells separated by thick fibrous septa. The fibrous stroma showed various degree of lymphoplasmacytic infiltration, resulting in a prominent lymphoepithelioma-like pattern in 3 cases. Two cases showed squamoid differentiation, mimicking thymic Hassall corpuscles. Immunohistochemically, the tumor was consistently positive for cytokeratins, CD5, bcl-2, p63 and CD117. CEA was variably expressed. The staining for thyroglobin and TTF1 was negative. There was no labeling for EBER in all the cases. Two patients experienced local recurrence at 22 months and 12 years after surgery, respectively. They were treated with re-resection. All patients remained well on follow up. The duration of follow up ranged from 4 to 55 months.

CONCLUSIONSCASTLE is a low-grade thyroid carcinoma with the morphologic features and immunophenotypes overlapping with those of thymic carcinoma. Awareness of this rare entity is important to both the pathologists and clinicians.