A 52-year-old woman undergoes thyroidectomy for a thyroid nodule. Histopathology shows a tumor composed of sheets of cells with abundant cytoplasm, round nuclei, and amyloid deposition. Immunohistochemistry is positive for calcitonin and chromogranin A. All of the following are true regarding this tumor EXCEPT:

Explanation

## Medullary Thyroid Carcinoma (MTC) — Clinical & Pathological Features ### Case Analysis The histopathology and immunohistochemistry findings are pathognomonic for **medullary thyroid carcinoma**: - Sheets of cells with abundant cytoplasm → neuroendocrine morphology - Amyloid deposition → composed of calcitonin protein - Calcitonin and chromogranin A positivity → neuroendocrine marker confirmation ### Origin & Embryology **Key Point:** Medullary thyroid carcinoma arises from **parafollicular C cells**, which are derived from the **neural crest** (specifically, the ultimobranchial body). - Distinct from follicular epithelium-derived carcinomas (papillary, follicular, anaplastic) - Produces neuroendocrine markers: calcitonin, carcinoembryonic antigen (CEA), chromogranin A ### Tumor Markers & Surveillance **High-Yield:** Calcitonin is the **gold standard marker** for medullary thyroid carcinoma. - Serum calcitonin is highly sensitive and specific - Used for diagnosis and post-operative surveillance - Calcitonin doubling time predicts prognosis - CEA levels also elevated; CEA/calcitonin ratio helps assess differentiation ### Molecular Genetics **Mnemonic:** **MEN 2 = Medullary + Pheochromocytoma + Parathyroid** - **Familial MTC** (20–25% of cases) associated with **RET proto-oncogene mutations** - MEN 2A: MTC + pheochromocytoma + primary hyperparathyroidism - MEN 2B: MTC + pheochromocytoma + mucosal neuromas + marfanoid habitus - Sporadic MTC (75% of cases): somatic RET mutations ### Treatment & Radioactive Iodine Responsiveness **Warning:** This is the key differentiator. Medullary thyroid carcinoma is **NOT responsive to radioactive iodine (I-131)**. - MTC does NOT take up iodine because it arises from non-follicular C cells - C cells do not concentrate iodine (unlike follicular cells) - Treatment: surgical resection, external beam radiation, chemotherapy (vandetanib, cabozantinib for advanced disease) - ~~Radioactive iodine therapy~~ is ineffective and should not be used ### Comparison: Iodine Responsiveness | Tumor Type | Iodine Uptake | I-131 Therapy | |------------|---------------|---------------| | Papillary thyroid carcinoma | Yes | Yes | | Follicular thyroid carcinoma | Yes | Yes | | Medullary thyroid carcinoma | **No** | **No** | | Anaplastic thyroid carcinoma | No | No | **Clinical Pearl:** A patient with elevated calcitonin and a thyroid nodule should undergo RET mutation testing and screening for MEN 2 syndrome (plasma free metanephrines, serum calcium, imaging). Family members of RET-positive patients require prophylactic thyroidectomy in childhood. ### Summary of Correct Statements 1. ✓ Neural crest origin — **TRUE** (parafollicular C cells) 2. ✓ Calcitonin as tumor marker — **TRUE** (sensitive and specific) 3. ✗ Radioactive iodine responsiveness — **FALSE** (MTC does not concentrate iodine) 4. ✓ RET mutations in familial MTC — **TRUE** (MEN 2 syndromes)