A 52-year-old man with a history of medullary thyroid carcinoma undergoes genetic testing and is found to have a RET proto-oncogene mutation. Which of the following is the most common site of metastasis in medullary thyroid carcinoma?

Explanation

## Medullary Thyroid Carcinoma: Metastatic Pattern **Key Point:** Regional lymph nodes (central and lateral cervical nodes) are the most common site of metastasis in medullary thyroid carcinoma (MTC), occurring in 50% of patients at presentation. ### Metastatic Spread Pattern in MTC | Site of Metastasis | Frequency at Diagnosis | Timing | | --- | --- | --- | | Regional lymph nodes (cervical) | 50% | Early | | Liver | 10–15% | Intermediate | | Lung | 5–10% | Intermediate to late | | Bone | 5% | Late | | Brain | <1% | Very late | | Adrenal glands | Rare | Late | **High-Yield:** MTC is unique among thyroid cancers because it spreads to *regional lymph nodes first*, then to distant organs (liver > lung > bone). This is different from papillary carcinoma (which also spreads to lymph nodes early) and follicular carcinoma (which tends toward hematogenous spread to lung/bone). ### Biology of MTC Metastasis 1. **Lymph node predilection**: - MTC arises from parafollicular C cells (neuroendocrine origin) - Early lymphatic invasion is characteristic - Central compartment nodes involved first (level VI) - Lateral compartment nodes involved later (levels II–V) 2. **Distant metastasis sequence**: - Liver (most common distant organ, 10–15%) - Lung (5–10%) - Bone (5%) - Brain, adrenal, kidney (rare) **Clinical Pearl:** Calcitonin and carcinoembryonic antigen (CEA) levels correlate with tumour burden and metastatic disease. A doubling time of calcitonin <6 months suggests aggressive disease with high risk of distant metastases. ### RET Mutation & Aggressiveness **Mnemonic — MTC Risk Stratification (ATA 2015): ATA Level** - **A** (Lowest risk): M918T absent, normal calcitonin, no lymph node involvement - **B** (Intermediate): M918T present OR elevated calcitonin OR lymph node involvement - **C** (Highest risk): Distant metastases, very high calcitonin RET M918T mutations (most common in sporadic MTC) are associated with more aggressive disease and earlier lymph node metastasis. ### Why NOT Distant Sites First? - **Lung and bone** are common in follicular and anaplastic cancers (hematogenous spread), not MTC - **Brain metastases** are extremely rare in MTC and occur only in advanced disease - **Adrenal glands** may be involved in MEN 2A/2B syndrome (pheochromocytoma coexistence) but are not a primary metastatic site of MTC itself **Warning:** Do not confuse MTC metastatic pattern with other thyroid cancers. Papillary carcinoma also spreads to lymph nodes early, but MTC has a *higher* propensity for lymph node involvement (50% vs. 30% in PTC) and a *lower* propensity for distant metastases initially. [cite:Robbins 10e Ch 24]