Answer: Metastatic cystic papillary carcinoma

Histology: none provided

Discussion: Sections show thymic tissue with adjacent cystic lymph nodes. The cyst is lined by a single layer of cuboidal or flattened epithelium with focal minute papillary formations. Cytologically, the cuboidal cells show nuclear clearing and grooves. The lumen is filled by homogenous pink proteinaceous material consistent with colloid. Focally, follicles are also noted within the lymph node parenchyma. This cystic lymph node is surrounded by histologically unremarkable thymus. Immunostains demonstrate that the lining cells are strongly TTF-1 and thyroglobulin consistent with a metastatic cystic papillary carcinoma of the thyroid. Thyroglossal duct remnants can become cystic and are typically lined by pseudostratified ciliated or squamous epithelium. Mucous glands and thyroid follicles can often be seen in the adjacent stroma. The thyroid tissue can undergo malignant transformation usually in the form of papillary carcinoma. Ectopic thymic tissue can often be found in the neck and occasionally may be cystic. Ectopic thymic tissue is usually located adjacent to the thyroid and/or parathyroid but can also be seen in the skin of the neck. They are characteristically lined by flattened or squamous epithelium that is cytokeratin positive. Branchial cleft cysts are not related to the thyroid and are typically lateral neck lesions. They are usually lined by squamous or ciliated columnar epithelium and are associated with abundant lymphoid tissue.