A 52-year-old man with suspected pheochromocytoma presents with episodic hypertension, palpitations, and profuse sweating. Plasma metanephrines are elevated. Which investigation is the gold standard to localize the tumor before surgical planning?

Explanation

## Investigation of Choice for Pheochromocytoma Localization **Key Point:** ¹²³I-MIBG (metaiodobenzylguanidine) scintigraphy is the gold standard functional imaging modality for pheochromocytoma localization and detection of metastases. ### Why ¹²³I-MIBG is Superior **High-Yield:** MIBG is a noradrenaline analog that is taken up by chromaffin cells via the noradrenaline transporter (NET), making it highly specific for neuroendocrine tumors. It detects: - Primary tumors (>90% sensitivity) - Extra-adrenal paragangliomas - Metastatic disease - Bilateral adrenal involvement **Clinical Pearl:** While CT/MRI show anatomy, MIBG shows **function** — it identifies catecholamine-secreting tissue regardless of size or location. This is critical for: - Detecting small tumors (<2 cm) - Finding extra-adrenal paragangliomas (10–15% of cases) - Identifying metastases to bone, liver, or distant sites ### Comparison of Imaging Modalities | Investigation | Sensitivity | Specificity | Best For | Limitation | | --- | --- | --- | --- | --- | | ¹²³I-MIBG | 90–95% | 99% | Functional localization, metastases | Requires nuclear medicine facility | | CT | 85–90% | 70–80% | Anatomic detail, size assessment | Misses small/extra-adrenal tumors | | MRI T2 | 85–90% | 70–80% | Soft tissue contrast | No functional information | | Ultrasound | 60–70% | 50–60% | Screening only | Operator-dependent, poor sensitivity | **Mnemonic:** **MIBG = Most Important for Biochemically-confirmed Gland** — use it once biochemical diagnosis (elevated metanephrines) is confirmed. ### Clinical Algorithm ```mermaid flowchart TD A[Elevated plasma metanephrines]:::outcome --> B[Pheochromocytoma suspected] B --> C{Localization needed?}:::decision C -->|Yes| D[¹²³I-MIBG scintigraphy]:::action D --> E[Detects primary + metastases]:::outcome E --> F[Surgical planning]:::action C -->|Contraindication to MIBG| G[CT or MRI for anatomy]:::action ``` **Tip:** MIBG is ordered AFTER biochemical confirmation (24-h urine metanephrines or plasma free metanephrines). Do not skip biochemistry — imaging without biochemical proof is wasteful. [cite:Harrison 21e Ch 405]