A 42-year-old woman with hypertension and family history of MEN 2A syndrome has elevated plasma free metanephrines (8× ULN). CT abdomen shows a 2.5 cm right adrenal mass. Which is the most appropriate next investigation to confirm the diagnosis and assess for metastatic disease?

Explanation

## Functional Imaging After Biochemical and Anatomical Confirmation **Key Point:** I-123 MIBG scintigraphy is the most appropriate next investigation after biochemical confirmation and anatomical localization of a pheochromocytoma because it: 1. Confirms the functional nature of the lesion (catecholamine-secreting). 2. Detects metastatic or extra-adrenal disease (sensitivity ~90%). 3. Guides surgical planning and risk stratification. ### Role of MIBG Scintigraphy in Pheochromocytoma Workup **High-Yield:** MIBG is a noradrenaline analog that is taken up by chromaffin tissue (adrenal medulla and pheochromocytoma cells via noradrenaline transporter). It is the functional imaging modality of choice for: - Confirming the catecholamine-secreting nature of the adrenal mass. - Detecting metastatic disease (bone, liver, lungs, lymph nodes). - Identifying extra-adrenal pheochromocytomas (paragangliomas). - Assessing for bilateral adrenal involvement (important in MEN 2A). ### MIBG vs. PET-CT | Feature | I-123 MIBG | F-18 FDG-PET-CT | F-18 DOPA-PET-CT | | --- | --- | --- | --- | | Sensitivity | 90% | 85–95% | 95–98% | | Specificity | 95% | 60–70% | 95% | | Availability | Widely available | Limited | Very limited | | Cost | Lower | Higher | Higher | | Metastasis detection | Excellent | Good | Excellent | | Recommended use | First-line functional imaging | Alternative if MIBG unavailable | Research/specialized centers | **Clinical Pearl:** In MEN 2A patients, MIBG scintigraphy is particularly valuable because: - 10–15% have bilateral pheochromocytomas. - Extra-adrenal paragangliomas occur in ~5% of MEN 2A cases. - Early detection of metastatic disease influences surgical strategy. ### Diagnostic Algorithm After Biochemical Confirmation ```mermaid flowchart TD A[Elevated plasma free metanephrines]:::outcome --> B[Anatomical imaging: CT/MRI abdomen]:::action B --> C{Mass identified?}:::decision C -->|Yes, localized| D[Functional imaging: MIBG scintigraphy]:::action C -->|No| E[Repeat imaging or consider extra-adrenal disease]:::action D --> F{MIBG positive?}:::decision F -->|Yes| G[Confirms pheochromocytoma; assess for metastases]:::outcome F -->|No| H[Consider alternative diagnosis]:::outcome G --> I[Surgical planning]:::action ``` **Mnemonic:** **MIBG-MAP** — MIBG for Metastasis Assessment and Planning (after biochemical and anatomical confirmation).