Papillary thyroid carcinoma in a branchial cleft cyst.

Summary: Papillary thyroid carcinoma (PTC) in a branchial cleft cyst (BCC) is exceedingly rare. This case report describes a 53-year-old male with a BCC containing PTC. Despite normal preoperative thyroid imaging, total thyroidectomy revealed multifocal bilateral micropapillary thyroid carcinoma with a contralateral metastatic lymph node, suggesting an aggressive disease course. This finding supports the likelihood that thyroid gland carcinoma metastasized to the BCC. However, the possibility of a primary tumor arising from ectopic thyroid tissue within the cyst cannot be excluded. To our knowledge, this is the 11th reported case of papillary thyroid cancer noted in a BCC. The patient underwent successful treatment, including radioiodine ablation, highlighting the importance of thorough diagnostic evaluation and management in such rare presentations. Learning points: While most BCCs are benign, PTC can very rarely present within BCC. Metastatic PTC with a necrotic lymph node is often misdiagnosed as BCC, both radiographically and histologically. Since PTC arising as a primary tumor from ectopic thyroid tissue within a BCC is extremely rare, total thyroidectomy should be considered even in patients with radiographically normal thyroid to rule out micropapillary primary thyroid tumors. A thyroglobulin assay from the needle washout of a fine-needle aspiration of a BCC may help preoperatively identify differentiated thyroid cancers. Micropapillary thyroid cancers (<1 cm) are usually indolent, but some may show nodal metastases and clinical progression.