Answer: Lymphoepithelial cyst

Histology: Gross examination of the thyroid resection specimen revealed a 3 cm unilocular cyst located in the right lobe. Microscopically, the cyst is lined by a stratified squamous epithelium. The cyst wall contains a dense lymphoid infiltrate with the formation of germinal centers. The surrounding thyroid shows changes of florid Hashimoto’s thyroiditis.

Discussion: Intrathyroidal lymphoepithelial cyst is a rare cause of nodular thyroid enlargement. The etiology of the cyst formation is not known, although some have suggested origin of these branchial cleft-like lesions from branchial cleft derivatives in the thyroid. Enlargement is likely induced by immunologic mechanisms as these cysts are nearly always encountered in a background of chronic lymphocytic thyroiditis. Unlike lymphoepithelial cysts in the parotid and thymus, their presence in the thyroid is not associated with HIV infection, even when multifocal.

Lymphoepithelial cysts of the thyroid may be solitary or multifocal. They appear as hypoechoic masses on ultrasound. Fine needle aspiration usually reveals reactive lymphocytes without any more specific findings.

Histologically, they are essentially identifical to branchial cleft cysts located laterally in the neck. The lining epithelium may show any combination of squamous cells, cuboidal cells and even columnar cells with cilia. The most important entity to rule out of the differential diagnosis is a cystic papillary carcinoma where the tumor cells have undergone extensive squamous metaplasia. Thorough histologic examination of the entire cyst wall in an effort to identify any focal areas of conventional papillary carcinoma is advised.