A 68-year-old woman has a rapidly enlarging fixed neck mass compressing the trachea, developing over six weeks. Biopsy shows sheets of highly pleomorphic giant cells and spindle cells with numerous mitoses. Immunohistochemistry shows cytokeratin positivity but thyroglobulin and TTF-1 negativity. The most likely diagnosis is:
- A Poorly differentiated (insular) thyroid carcinoma
- B Metastatic melanoma to the thyroid
- C Anaplastic thyroid carcinoma ✓
- D Primary thyroid lymphoma
Correct answer: C. Anaplastic thyroid carcinoma
Explanation
Rapid growth in an elderly patient with pleomorphic giant and spindle cells defines anaplastic (undifferentiated) thyroid carcinoma. It retains epithelial lineage markers such as cytokeratin but loses differentiation markers, so thyroglobulin and often TTF-1 are negative, which distinguishes it from poorly differentiated carcinoma where thyroglobulin expression persists. Melanoma would express S100, SOX10 and HMB45, and lymphoma would be CD45 positive and cytokeratin negative.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.