A 68-year-old man presents with a 3-week history of rapidly enlarging anterior neck mass, dysphagia, and stridor. On examination, the thyroid is hard, fixed, and irregular. Biopsy shows sheets of atypical cells with marked nuclear pleomorphism, high mitotic rate, and areas of necrosis. Immunohistochemistry is positive for p53 and Ki-67 (>30%). What is the most likely diagnosis?

Explanation

## Diagnosis: Anaplastic Thyroid Carcinoma (ATC) ### Clinical Presentation **Key Point:** Anaplastic thyroid carcinoma is the most aggressive thyroid malignancy, presenting with rapid growth, airway obstruction, and early distant metastases in elderly patients. The clinical vignette is classic for ATC: - **Age:** 68 years (typically >60 years) - **Presentation:** Rapid enlargement over weeks (not months/years) - **Symptoms:** Dysphagia, stridor (airway involvement) - **Exam:** Hard, fixed, irregular mass (locally invasive) ### Pathological Features | Feature | ATC | PDC | MTC | Lymphoma | |---------|-----|-----|-----|----------| | **Differentiation** | Undifferentiated | Partially differentiated | Differentiated (C-cell origin) | Non-epithelial | | **Nuclear pleomorphism** | Marked | Moderate | Mild to moderate | Variable | | **Mitotic rate** | Very high (>10/HPF) | High (5–10/HPF) | Low to moderate | Variable | | **Necrosis** | Extensive | Present | Absent | Variable | | **Ki-67 index** | >30% (often >50%) | 10–30% | <10% | Variable | | **p53 overexpression** | Common | Uncommon | Rare | Uncommon | ### Histopathological Features of ATC **High-Yield:** ATC shows complete loss of follicular differentiation with spindle cell, giant cell, or squamoid patterns. 1. **Morphology:** - Sheets of atypical cells (no follicular structures) - Spindle cell, giant cell, or mixed patterns - Marked nuclear pleomorphism - Abundant mitotic figures (including abnormal forms) - Extensive tumor necrosis 2. **Immunohistochemistry:** - **Positive:** PAX8 (70–80%), p53 (overexpression), Ki-67 (>30%, often >50%) - **Negative or weak:** Thyroglobulin, calcitonin, TTF-1 (may be lost) - **Markers of aggressive biology:** High p53, high Ki-67 3. **Molecular features:** - TP53 mutations (60–80%) - BRAF V600E mutations (25–40%) - RAS mutations (20–30%) - PTEN loss - Often arises from pre-existing differentiated thyroid cancer (PTC or FTC) ### Clinical Pearl **Clinical Pearl:** ATC frequently arises from dedifferentiation of a pre-existing papillary or follicular carcinoma. Always ask about prior thyroid disease or prior thyroid cancer diagnosis. ### Prognosis and Management **Warning:** ATC is one of the most lethal human malignancies with median survival of 3–6 months if untreated. - **Staging:** All ATCs are considered Stage IV (AJCC): - IVA: Intrathyroidal disease - IVB: Gross extrathyroidal extension - IVC: Distant metastases - **Metastatic pattern:** Lung (80%), bone (15%), brain (10%) - **Treatment:** Multimodal therapy (surgery + chemotherapy + radiation) for resectable disease; palliative chemotherapy for unresectable/metastatic disease ### Distinction from Poorly Differentiated Carcinoma (PDC) While both ATC and PDC show aggressive features, ATC is defined by **complete loss of follicular differentiation** with **marked nuclear pleomorphism** and **very high mitotic rate (>10/HPF)**. PDC retains some follicular architecture and shows less extreme nuclear atypia. [cite:Robbins 10e Ch 24]