Squamous cell components in a thyroid follicular adenoma: significant evidence of follicular cell origin by histomorphological, immunohistochemical and molecular analyses.

Squamous cells in the thyroid appear in a variety of conditions, including adenomatous goiters, malignant neoplasms, inflammatory diseases, and embryonic remnants. However the origin of the squamous cells is still under dispute. Here we report a case of an encapsulated follicular cell tumor consisting of follicular cells, basaloid squamous cells, and morphologically intermediate cells. The patient was a 66-year-old man presenting with a progressively enlarged painless lump in the right side of his neck. A solid tumor with encapsulation in the right lobe was confirmed by simple right lobe thyroid lobectomy. This tumor demonstrated heterogeneous immunoreactions for Ki-67, thyroglobulin, thyroid transcription factor-1, and pan-cytokeratin (AE1/AE3), in that the intermediate cells had intermediate immunoreactivity between follicular and basaloid squamous cells. p63 was positive in the periphery layer of tumor cell nests of the intermediate and squamous cells. A completely negative immunoreaction was noted for high molecular weight cytokeratin (34βE12), calcitonin, CEA, p53, CD5, and rearranged in transformation (RET). Mutational analysis of BRAF and RAS were negative. These results strongly suggest that this tumor is a follicular adenoma, and that the squamous component originated from follicular cells undergoing squamous metaplasia. The patient has been disease free more than 40 months after surgery.