Primary pure squamous cell carcinoma of the thyroid: Report and histogenic consideration of a case involving a BRAF mutation

Primary squamous cell carcinoma of the thyroid (SCC‐T) is extremely rare. Its clinical presentation is similar to that of anaplastic carcinoma. Metastasis or extension from the head and neck area should be ruled out, as patients with SCC‐T have a poorer prognosis than patients who have a thyroid extension from an adjacent tumor. An 87‐year‐old man presented with a longstanding painless mass in the right thyroid and had experienced 2 months of pain upon swallowing. A right lobectomy was performed with resection of thyroid cartilage, cricoid cartilage, a portion of the first to third tracheal ring and the right neck lymph node. A histological examination revealed pure SCC. The tumor cells showed diffuse immunoreactivity to CK5/6, CK19 and p63. Immunoreactivity to EMA and p53 was focally positive. TTF‐1, galectin 3 and thyroglobulin immunoreactivity was restricted to the non‐neoplastic thyroid tissue. Both tumor cells and non‐neoplastic follicular cells were negative for CD5. The MIB‐1 index was 36%. DNA extracted from the tumor identified a BRAF V600E mutation in exon 15 and a BRAF G468A mutation in exon 11, whereas DNA from non‐tumorous cells did not contain a mutation. These molecular findings may suggest a direct transformation from papillary carcinoma to SCC‐T.