What are the characteristic findings of pheochromocytoma on computed tomography (CT) scan?

Characteristic CT Findings of Pheochromocytoma

Pheochromocytomas typically present on CT scan with higher attenuation values (>10 HU) on non-contrast CT, heterogeneous enhancement after contrast administration, and may demonstrate delayed contrast washout that can mimic malignant lesions.

Key Imaging Features

Non-contrast CT Findings

• Higher attenuation values (>10 Hounsfield Units) on non-contrast CT 1
• Typically more dense than benign adrenal adenomas
• Approximately 30% of pheochromocytomas may demonstrate adenoma-like washout patterns, leading to potential misdiagnosis 2

Contrast-Enhanced CT Findings

• Heterogeneous enhancement pattern (seen in 75% of pheochromocytomas vs 51% of adenomas) 2
• Avid enhancement in portal venous phase
• Delayed washout of contrast (<60% absolute washout at 15 minutes) 3
• Variable relative percentage enhancement washout (RPEW), though typically lower than adenomas (37.4% vs 57.3%) 2

Morphological Features

• Necrosis is more common in larger pheochromocytomas (41.6% in larger tumors vs 12.5% in smaller ones) 2
• Size is variable but often >3 cm
• May have well-defined borders, though irregular margins can occur
• Rarely contains calcifications

Diagnostic Pitfalls

1. Washout Characteristics Overlap: About 50% of pheochromocytomas may demonstrate washout patterns similar to benign adenomas, particularly those <3 cm 2

2. Size Considerations: While size alone is not diagnostic, pheochromocytomas are often larger than incidental adenomas

   • Necrosis is more common in larger pheochromocytomas 2

3. False Positives on Functional Imaging: CT-based attenuation correction on SPECT/CT can lead to enhanced physiological visualization of the adrenal medulla, potentially causing false-positive interpretations 1

Diagnostic Algorithm

1. Initial Evaluation: Non-contrast CT to assess Hounsfield Units (HU)

   • HU >10 raises suspicion for pheochromocytoma or other non-adenomatous lesions 3

2. Second-Line Imaging: For indeterminate lesions

   • Contrast-enhanced washout CT or chemical shift MRI 3
   • Assess for heterogeneous enhancement and delayed washout patterns

3. Functional Imaging when pheochromocytoma is suspected:

   • 123I-MIBG scintigraphy has high specificity (95-100%) for pheochromocytoma 1
   • Consider FDG-PET for potentially malignant lesions 3

Clinical Correlation

• Always correlate imaging findings with biochemical testing (plasma free metanephrines or 24-hour urinary metanephrines) 1, 3
• Pheochromocytoma should be excluded before any adrenal biopsy is considered 1
• In patients with NF1, pheochromocytomas are exclusively adrenal (no paragangliomas), with 20% being multifocal and 12% malignant 1

Remember that while CT findings are highly suggestive, the definitive diagnosis of pheochromocytoma requires biochemical confirmation of catecholamine excess, as approximately 1.5-14% of incidentally discovered adrenal masses are pheochromocytomas 1.