A 68-year-old woman notices rapid enlargement of a long-standing thyroid nodule over six weeks, with hoarseness and dysphagia. The mass is hard and fixed. Biopsy shows sheets of highly pleomorphic giant cells and spindle cells with numerous mitoses; some areas resemble squamous differentiation. Immunostains for thyroglobulin and calcitonin are negative. The most likely diagnosis is:

• A Medullary thyroid carcinoma
• B Diffuse large B-cell lymphoma arising in Hashimoto thyroiditis
• C Anaplastic (undifferentiated) thyroid carcinoma ✓
• D Poorly differentiated (insular) thyroid carcinoma

Explanation

Rapidly enlarging fixed thyroid mass in an elderly patient with pleomorphic giant and spindle cells is classic anaplastic carcinoma, typically arising from pre-existing differentiated carcinoma. Negative thyroglobulin and calcitonin rules out medullary carcinoma, which is also positive for calcitonin and chromogranin. Lymphoma would show CD20-positive lymphoid cells rather than pleomorphic epithelial giants, and insular carcinoma retains some follicular differentiation.

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.