Ectopic Thyroid Carcinoma Presenting as a Superior Mediastinal Cystic Mass

Ectopic thyroids are thyroid tissue that develops in locations other than its normal position. During their developmental process, they rarely form outside the route of the thyroglossal duct, especially below the normal thyroid gland position. Most ectopic thyroids have normal histological findings, and malignant tumors are relatively rare. In this report, we present a case of ectopic thyroid carcinoma that presented as a mediastinal cystic lesion. A 46-year-old woman was incidentally diagnosed with a mediastinal cystic mass during a follow-up for another condition. The mass was surgically resected, and pathological examination revealed malignant cells in the small papillary area and positive staining of the superficial epithelium for thyroid transcription factor-1 using immunochemistry. An orthotopic normal thyroid gland without tumor lesions was observed on CT and MRI, leading to a diagnosis of papillary adenocarcinoma originating from an ectopic thyroid in the mediastinum. Although it must always be differentiated from microcarcinoma metastasis, ectopic thyroid carcinoma should be considered in the differential diagnosis of cystic lesions in the mediastinum. The follow-up and treatment strategies for ectopic thyroid carcinoma are controversial and require individualized consideration for each case.