A 35-year-old woman with a family history of medullary thyroid carcinoma (MTC) presents for screening. She is asymptomatic with normal thyroid examination. Serum calcitonin is elevated at 45 pg/mL (normal <10 pg/mL). Which investigation is most appropriate as the next step to localize the MTC?

Explanation

## Investigation of Choice for MTC Localization ### Clinical Context This patient has a family history of MTC and an elevated serum calcitonin (45 pg/mL), confirming the diagnosis of medullary thyroid carcinoma. The next step is **localization** of the primary tumor and assessment of extent. ### Why High-Resolution Ultrasound is the First-Line Imaging **Key Point:** High-resolution ultrasound (HRUS) of the thyroid is the **first-line imaging modality** for localizing MTC and assessing thyroid nodules in patients with elevated calcitonin. **High-Yield:** Calcitonin is the **gold standard tumor marker for MTC diagnosis and monitoring**. Once elevated calcitonin confirms MTC, imaging is needed to: 1. Localize the primary tumor in the thyroid 2. Assess for multifocal disease 3. Evaluate cervical lymph nodes 4. Guide further staging and management **Mnemonic: HRUS for MTC** — **H**igh-resolution **U**ltrasound is the **R**ight **S**tart for MTC localization. ### Imaging Modalities for MTC Staging | Modality | Role | Sensitivity | Timing | | --- | --- | --- | --- | | High-resolution ultrasound (HRUS) | Primary thyroid localization, cervical nodes | 80–90% for nodules >5 mm | First-line | | CT neck/chest | Cervical and mediastinal nodes, staging | Moderate | If HRUS shows nodule or nodes | | MRI neck | Soft tissue detail, nodal assessment | High | Alternative to CT if renal disease | | FDG-PET | Distant metastases (bone, liver, lung) | 40–60% for MTC | Only if calcitonin >500 pg/mL or CEA elevated | | Calcitonin stimulation test | Diagnosis confirmation (not localization) | N/A | Rarely needed if baseline calcitonin elevated | **Clinical Pearl:** In MTC, **calcitonin level correlates with tumor burden**. A calcitonin of 45 pg/mL suggests early/small disease, making HRUS ideal for detecting small nodules. FDG-PET has poor sensitivity for MTC and is reserved for advanced disease with very high calcitonin (>500 pg/mL) or elevated CEA. ### Role of Other Investigations **Serum CEA:** A secondary tumor marker in MTC used for **prognosis and monitoring**, not localization. CEA is less sensitive and specific than calcitonin. It is elevated in ~50% of MTC cases and correlates with tumor burden and metastatic disease. **Thyroid FNAC:** Not indicated when calcitonin is already elevated and diagnostic. FNAC is used to evaluate thyroid nodules of uncertain significance, not to confirm or localize known MTC. **FDG-PET:** Has poor sensitivity for primary MTC (40–60%) and is reserved for patients with very high calcitonin levels (>500 pg/mL) suggesting advanced/metastatic disease. In this patient with calcitonin 45 pg/mL, FDG-PET is premature and unnecessary. ### Management Algorithm ```mermaid flowchart TD A[Elevated serum calcitonin]:::outcome --> B[Diagnosis: MTC confirmed]:::outcome B --> C[High-resolution ultrasound of thyroid]:::action C --> D{Nodule/nodes detected?}:::decision D -->|Yes| E[CT neck/chest for staging]:::action D -->|No| F[Repeat HRUS + calcitonin in 3-6 months]:::action E --> G{Calcitonin >500 or CEA elevated?}:::decision G -->|Yes| H[FDG-PET for distant metastases]:::action G -->|No| I[Proceed to thyroidectomy]:::action ``` [cite:Harrison 21e Ch 297; Robbins 10e Ch 24]