Thyroid papillary carcinoma in lateral neck cyst: missed primary tumour or ectopic thyroid carcinoma within a branchial cyst?

We report a case of thyroid papillary carcinoma lying within a lateral cervical cyst for which no occult primary tumour of the thyroid was identified. We explore the possible diagnoses and include a discussion of how ectopic thyroid tissue may come to lie within a branchial cyst.