Case 133

A 73-year-old female with a 7.8 cm large thyroid mass.

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1.What's the diagnosis?

A: Papillary thyroid carcinoma

B: Anaplastic thyroid carcinoma

C: Squamous cell carcinoma

D: Follicular carcinoma

Answer

This case is an anaplastic thyroid carcinoma.

Table: Overview of Anaplastic Thyroid Carcinoma (ATC)

Category	Feature	Details
Histopathology	Infiltrative pattern	Frequent extrathyroidal extension
	Necrosis	Common
	Mitotic activity	Prominent, including atypical mitoses
	Nuclear pleomorphism	Significant
	Angiolymphatic invasion	Frequent
	Morphologic variants	Epithelioid, spindle, giant cell, pleomorphic, paucicellular, rhabdoid, angiomatoid, squamous
	Special features	May show metaplastic chondroid or osteoid; neutrophilic infiltrate
Squamous Pattern of ATC	Morphology	Focal or diffuse squamous differentiation
	Reclassification	Primary thyroid squamous carcinoma now seen as a pattern of ATC
	Prognosis	Similarly poor to conventional ATC
	Association	Higher rate of associated differentiated carcinoma (76% vs 55%; P = 0.004)
	Molecular profile	Frequent BRAF p.V600E mutations
Immunohistochemistry	Cytokeratin	Positive in ~75%
	TTF1	Focal expression in a minority
	Thyroglobulin	Almost always negative; rare focal positivity in epithelioid areas
	PAX8 (MRQ-50)	Positive in ~50%; linked to epithelial morphology and coexisting DTC
	p53	Positive in >50%
	Ki-67 index	Usually very high
	Squamous-type ATC IHC	Positive for HMW keratins, 34βE12, CK5/6, p63, p40; occasional TTF1
	PAX8 & p53	More frequently positive in squamous-type ATC
Differential Diagnosis	Sarcomas	Synovial sarcoma, MPNST, angiosarcoma, leiomyosarcoma, rhabdomyosarcoma
	Other mimics	Melanoma, medullary thyroid carcinoma, mucoepidermoid carcinoma, Riedel thyroiditis, NUT carcinoma, epithelial thymoma, SETTLE
	Distinction from high-grade follicular-derived carcinomas	ATC shows marked pleomorphism, absence of organoid pattern, and lacks thyroglobulin expression

Case credit: UCSD Pathology

Author: Wangpan Jackson Shi, MD

Reference: https://tumourclassification.iarc.who.int/chaptercontent/53/48