Question

Which of the following imaging modalities is most sensitive for evaluation of extra-adrenal pheochromocytoma?

• A. USG
• B. CT
• C. MRI
• D. MIBG scan

Hint:

Extra-adrenal or multiple pheochromocytomas need a whole-body, tissue-specific scan, not just a regional anatomical one.

Answer 1

The Correct Option is D

Step 1: Understand extra-adrenal pheochromocytoma.
Pheochromocytomas that arise outside the adrenal gland are called extra-adrenal paragangliomas. They can sit anywhere along the sympathetic chain, from the neck down to the pelvis, and are more often multiple, bilateral, or malignant than the typical adrenal tumor.

Step 2: Recall why anatomical imaging can miss them.
USG, CT, and MRI are anatomical modalities. They show a mass if it happens to be in the field examined, but a small or ectopically placed extra-adrenal tumor away from the adrenal bed is easy to miss on a routine scan.

Step 3: Recall how MIBG works.
MIBG, metaiodobenzylguanidine, is structurally similar to norepinephrine and is taken up by chromaffin tissue through the same noradrenaline transporter. When labeled with iodine-131 or iodine-123, it gives a whole-body functional scan that lights up any tissue actively taking up the tracer, wherever it is located.

Step 4: Apply this to extra-adrenal disease.
Because MIBG scanning covers the whole body and is tissue-specific for chromaffin cells, it picks up ectopic, multiple, or metastatic deposits of pheochromocytoma that a regional CT or MRI could miss. This makes it the single most sensitive test for extra-adrenal disease among the options given.

Step 5: Note the current picture.
Today, functional PET tracers such as gallium-68 DOTATATE are increasingly used and can outperform MIBG in some extra-adrenal and metastatic cases, but among USG, CT, MRI, and MIBG, MIBG remains the correct and most sensitive choice.

Step 6: Final answer.
\[ \boxed{\text{MIBG scan}} \]