A 60-year-old man with a long-standing thyroid nodule develops rapid enlargement over 4 weeks with hoeness of voice. FNA shows highly pleomorphic spindle and giant cells with marked nuclear atypia. Immunohistochemistry is positive for cytokeratin and p53, and NEGATIVE for PAX8 and TTF-1. Which is the most likely diagnosis?
- A Sarcoma of the thyroid
- B Medullary thyroid carcinoma with sarcomatoid change
- C Anaplastic thyroid carcinoma ✓
- D Metastatic melanoma to thyroid
Correct answer: C. Anaplastic thyroid carcinoma
Explanation
Anaplastic thyroid carcinoma is a highly aggressive undifferentiated tumor of follicular epithelium. It often loses PAX8 and TTF-1 expression while retaining cytokeratin positivity, with p53 overexpression or mutation common. The rapid growth in a long-standing nodule with vocal cord palsy is classic. Sarcomas are cytokeratin negative. MTC expresses calcitonin. Melanoma expresses S100 and Melan-A, not cytokeratin.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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