The lymphoepithelial cyst (LEC) rarely occurs in the thyroid gland. The LEC has been thought to be related to developmental rest, namely solid cell nest, which is derived from ultimobranchial body. We report a case of lymphoepithial cyst in a 34- year-old woman clinically diagnosed with Hashimoto's thyroiditis. The cyst was located in mid to lower portion of the left lobe. It was a single unilocular cyst, which for the most part was lined with squamous epithelium, and at certain foci with ciliated columnar epithelium. The cyst wall showed a dense lymphocytic infiltration, numerous lymphoid follicles with germinal centers and admixed thyroid follicles. This morphology is similar to the branchial cleft cyst, with the exception of the thyroid follicles in the cyst wall. Near the cyst were several solid epidermoid cell nests. Immunohistochemical stain of this cyst-lining epithelium and solid cell nests showed CEA positivity. In view of the similarity in histomorphology and CEA positivity to branchial cleft cyst of the lateral neck, the LEC of the thyroid could also have been of branchial origin. However, the admixed thyroid follicles in the cyst wall suggests that the LEC of the thyroid gland might have derived from another branchial cleft as a ultimobranchial body, because it has the potential for thyroid follicular differentiation.
Branchial Region* ; Branchioma ; Epithelium ; Female ; Germinal Center ; Humans ; Neck ; Thyroid Gland* ; Thyroiditis ; Ultimobranchial Body