Table 2.

Differential diagnoses of squamoid category.

Thyroid lesions simulating a squamoid pattern	Differential features
Pure squamous cells carcinoma	(i) Entirely composed of squamous cells; (ii) no evidence of another type of thyroid carcinoma in close proximity.
Primary head and neck squamous cell carcinoma	(i) Presence of in situ component in head and neck structures; (ii) features of “ab extrinseco” involvement of thyroid parenchyma; (iii) PAX8 is consistently negative.
Metastatic squamous cell carcinoma of the lung	(i) Presence of a lung nodule clinically detected; (ii) PAX8 is consistently negative.
Diffuse sclerosing variant of papillary thyroid carcinoma	(i) Abundant psammoma bodies; (ii) neoplastic cells retain nuclear features of PTC; (iii) immunoreactivity for thyroglobulin.
Mucoepidermoid carcinoma	(i) Combination of squamoid and mucinous components; (ii) thought to represent papillary carcinoma with extreme degree of squamous and mucinous metaplasia; (iii) low grade thyroid neoplasm.
Sclerosing mucoepidermoid carcinoma with eosinophilia	(i) Fibrohyaline stroma; (ii) striking infiltration of eosinophils; (iii) mucin secretion is often present; (iv) typically arising in Hashimoto thyroiditis (thought to derive from metaplastic squamous nests associated with inflammatory infiltrate).
CASTLE (carcinoma with thymus-like elements)	(i) Lymphoepithelioma-like carcinoma with foci of squamous differentiation; (ii) pushing margins; (iii) immunoreactivity for CD5, bcl-2, high molecular weight keratin, mcl-1 (thought to be ectopic thymic carcinoma arising from remnants of the branchial pouch); (iv) usually indolent behavior with tendency to late recurrences.