Metastatic anaplastic carcinoma of the thyroid mimicking squamous cell carcinoma: Report of a case of a challenging cytologic diagnosis

Background: The probability of anaplastic transformation in a differentiated thyroid carcinoma (DTC) is <2%. Admixture of DTC and undifferentiated carcinoma can occasionally be found in thyroidectomy specimens, or the undifferentiated tumor may develop in metastatic foci months or years after removal of the primary tumor. Case: Anaplastic transformation of a metastatic DTC was diagnosed at autopsy in a 11-year-old female. At the time of her total thyroidectomy for follicular carcinoma, she already had several lung nodules, which diminished in size upon ¹³¹I treatment. Five years later the patient developed a new, pleura-based lung mass. The mass extended along the paraspinal soft tissue and involved the kidney. Fine needle aspiration biopsy of the paraspinal mass resulted in a diagnosis of metastatic squamous cell carcinoma. Two months later the patient died. Histologic evaluation of the lung nodules and pleura-based mass revealed areas of follicular carcinoma intermixed with a various patterns of undifferentiated carcinoma. The paraspinal and kidney mass consisted entirely of anaplastic tumor, including areas of squamoid differentiation. Conclusion: In patients with a history of DTC who develop a rapidly growing mass elsewhere, a possibility of metastatic DTC with anaplastic transformation must be considered.