Branchial-like cysts of the thyroid associated with solid cell nests.

Presented herein is the case of a 65-year-old man with a 20 year history of thyroid hypofunction. On sonography a cystic lesion 4 x 4 x 5 cm in largest diameter was found, replacing most of the right lobe of the thyroid gland. Microscopically, the lesion was composed of labyrinth-like cystic structures (LCS) lined by a few layers of benign-appearing squamous cells and filled by mucinous material. Adjacent to the cyst walls were discontinuous patches of a lymphoid tissue, composed of haloed centrocyte-like cells or germinal centers mostly depleted of germinal cells. Additionally, there were numerous squamous cell nests equivalent to solid cell nests (SCN), all of which were surrounded by a similar-looking lymphoid tissue. Rare SCN were thus cystically changed and contained a small amount of mucus. The SCN communicated with the LCS: the former represented the most distal outpouchings of the latter. The epithelial structures were surrounded by a loose collagenous adipocytic stroma with plump fibroblasts, which resembled the stroma often seen in lateral neck cysts associated with structures such as cartilage, accessory salivary gland tissues, cysts and accessory thyroid and thymus. Immunohistochemically, all lesional elements were negative for calcitonin and thyroglobulin. The results of the paper suggest that branchial cleft-like cyst have an origin in the ultimobranchial body.