Update on Anaplastic Thyroid Carcinoma: Morphological, Molecular, and Genetic Features of the Most Aggressive Thyroid Cancer

Table 1

Differential diagnoses of sarcomatoid category.

Thyroid lesions simulating a sarcomatoid pattern		Differential features
Other malignancies	Primary or metastatic sarcoma	It is an exclusion diagnosis: (i) lack of a well-differentiated component; (ii) no epithelial markers; (iii) absence of palisading necrosis and neoplastic spindle cells infiltrating the wall of large-sized vessels; (iv) presence of extrathyroidal sarcoma clinically detected (in metastatic disease).
	SETTLE; (spindle epithelial tumor with thymus-like elements)	(i) Adolescent or young adults (mean age 15 years); (ii) biphasic pattern of growth with a predominant spindle cell component merging with mucin-secreting glandular elements; both components have an epithelial phenotype; (iii) generally indolent behavior.
	Spindle cell variant of papillary thyroid carcinoma	Metaplastic variant of PTC: (i) spindle cells retain even if focally nuclear features of PTC; (ii) consistent immunoreactivity for thyroglobulin.
	Spindle cell variant of medullary carcinoma	(i) Presence of amyloid deposits; (ii) immunoreactivity for calcitonin and/or calcitonin gene-related peptide.
Benign processes	Solitary fibrous tumor	(i) Low mitotic rate (4 mitoses or fewer per 10 high-power fields); (ii) no necrosis or vascular invasion; (iii) positivity for bcl-2, CD34, CD99, and vimentin and negativity for all epithelial markers.
	Riedel thyroiditis	(i) Absence of necrosis; (ii) evidence of occlusive phlebitis (no angioinvasion); (iii) absence of neoplasm; (iv) negativity for epithelial markers; (v) generally benign self-limiting disease.
	Post-fine-needle aspiration Spindle cell nodules of the thyroid	(i) History of FNA biopsy; (ii) size ranging from 3 to 10 mm; (iii) not encapsulated but relatively circumscribed and located mostly in the center of preexisting thyroid nodules; (iv) low mitotic rate; (v) immunoreactivity for smooth muscle actin (myofibroblastic origin).