Medullary thyroid carcinoma is associated with mutations in which proto-oncogene, and what is the biochemical marker used for diagnosis and surveillance?

Explanation

## Medullary Thyroid Carcinoma (MTC) — Genetics and Biochemical Markers **Key Point:** Medullary thyroid carcinoma arises from parafollicular C cells and is uniquely associated with RET proto-oncogene mutations. It is the only thyroid cancer with a reliable biochemical tumor marker. ### RET Proto-Oncogene and MTC **High-Yield:** RET (REarranged during Transfection) mutations are found in: - **~50% of sporadic MTC cases** - **Nearly 100% of familial MTC cases** (part of MEN 2A and MEN 2B syndromes) RET is a receptor tyrosine kinase. Gain-of-function mutations lead to constitutive activation and uncontrolled C cell proliferation. ### Biochemical Markers for MTC | Marker | Role | Clinical Use | |--------|------|---------------| | **Calcitonin** | Hormone produced by C cells; elevated in MTC | Primary diagnostic marker; >100 pg/mL highly suggestive of MTC | | **Carcinoembryonic antigen (CEA)** | Produced by MTC cells; less specific than calcitonin | Prognostic marker; elevated CEA suggests aggressive disease | | **Calcitonin stimulation test** | Pentagastrin or calcium gluconate stimulation | Increases calcitonin levels in MTC; used for early detection in at-risk families | | **Thyroglobulin** | Produced by follicular cells, not C cells | NOT useful for MTC; used for papillary and follicular carcinomas | **Clinical Pearl:** A calcitonin level >100 pg/mL in a patient with a thyroid nodule is virtually diagnostic of MTC. Baseline calcitonin >10 pg/mL warrants further investigation. ### MTC in MEN Syndromes ```mermaid flowchart TD A[RET Mutation Identified]:::outcome --> B{Codon Location?}:::decision B -->|Codon 634| C[MEN 2A]:::outcome B -->|Codon 918| D[MEN 2B]:::outcome B -->|Other codons| E[Familial MTC]:::outcome C --> F[MTC + Pheochromocytoma + Primary Hyperparathyroidism]:::action D --> G[MTC + Pheochromocytoma + Mucosal Neuromas + Marfanoid habitus]:::action ``` **Mnemonic:** **MEN 2 = RET** — Remember that MEN 2A and MEN 2B are caused by RET mutations. MTC is the most common and earliest manifestation in both syndromes. ### Why Other Options Are Wrong - **TP53:** Associated with Li-Fraumeni syndrome and anaplastic thyroid carcinoma, not MTC. - **BRAF:** Commonly mutated in papillary thyroid carcinoma (especially tall-cell variant), not MTC. - **PTEN:** Associated with Cowden syndrome and follicular thyroid carcinoma, not MTC. **Warning:** Do not confuse thyroglobulin (marker of follicular-derived cancers: papillary and follicular) with calcitonin (marker of C-cell-derived cancer: medullary). This is a high-yield trap. [cite:Robbins 10e Ch 24]