Squamous cell carcinoma of thyroid: a unique type of cancer in World Health Organization Classification.

The aim is to review the features of 117 primary squamous cell carcinomas of thyroid which meet the histological criteria of the World Health Organization classification of endocrine tumours. The carcinomas occur in 83 women and 34 men (female to male ratio is 2.4 to 1) and with median age at presentation of 64. Half of these squamous cell carcinomas of thyroid were moderately differentiated. PAX-8 protein is a sensitive marker for confirming the thyroid origin of the carcinoma. The carcinoma is also positive for p63, p40, cytokeratins 5/6, 7,19 and negative for cytokeratins 20 and 10/13. P53 overexpression is common. The most important differential diagnosis is direct infiltration or metastatic involvement by squamous cell carcinoma from other organs. Limited mutation analysis revealed BRAF mutation in some squamous cell carcinomas of the thyroid. The genetic profile appears to be different from anaplastic thyroid carcinomas. Primary squamous cell carcinoma of thyroid had lymph node involvement in 59% and distant metastases in 26%. The median survival of the patients was 8 months. Curative surgery offers the best survival for the patients with the carcinoma. To conclude, primary squamous cell carcinoma of the thyroid gland has distinctive clinical, pathological and molecular profiles. It is important to recognize this unique variant of thyroid carcinoma for possible curative surgical resection and to do more genomic works on the entity to uncover the molecular pathogenesis.