Washout Values for Pheochromocytoma vs Adenoma
Benign adenomas typically demonstrate ≥60% absolute washout or ≥40% relative washout on contrast-enhanced CT, but approximately one-third of pheochromocytomas also washout in this "adenoma range," making washout CT unreliable for distinguishing these lesions. 1, 2
Standard Washout Thresholds
For adenomas:
- Absolute percentage washout (APW) ≥60% 1, 3
- Relative percentage washout (RPW) ≥40% 1, 3
- These thresholds are considered diagnostic of lipid-poor adenoma 3
For pheochromocytomas:
- No specific diagnostic washout range exists, as these tumors demonstrate highly variable washout patterns 4, 5
Critical Limitation: Significant Overlap
The most important clinical caveat is that washout CT cannot reliably exclude pheochromocytoma:
- Approximately 33% of pheochromocytomas meet the washout criteria for adenoma (APW ≥60% or RPW ≥40%) 1, 2, 4
- In one study, 50% of pheochromocytomas demonstrated adenoma-like washout, with 80% of these being <3 cm in size 5
- Half of the pheochromocytomas that mimicked adenomas appeared homogeneous on all CT phases, further confounding diagnosis 4
- Some pheochromocytomas contain microscopic fat and can even demonstrate attenuation <10 HU on unenhanced CT, classically considered diagnostic of adenoma 6
Conversely, approximately one-third of benign adenomas fail to washout in the typical adenoma range (≥60% APW), meaning low washout does not confirm malignancy or pheochromocytoma. 1, 2
Additional Imaging Features That May Suggest Pheochromocytoma
While washout is unreliable, other CT characteristics may raise suspicion:
- Higher attenuation values: Pheochromocytomas typically have higher attenuation on unenhanced (mean ~26 HU), portal phase, and delayed phase imaging compared to adenomas, though overlap exists 5
- Heterogeneous enhancement: Only 25% of pheochromocytomas show homogeneous enhancement vs 49% of adenomas 5
- Presence of necrosis: 41.6% of larger pheochromocytomas contain necrosis vs only 3% of adenomas 5
- Larger size: Pheochromocytomas average 3.9 cm vs 2.0 cm for adenomas, though significant overlap exists 4
Mandatory Clinical Approach
Because imaging cannot reliably differentiate these lesions, biochemical testing with plasma or 24-hour urinary metanephrines is mandatory for ALL indeterminate adrenal masses before any intervention, regardless of washout values. 1, 2
Never proceed with biopsy or surgery without excluding pheochromocytoma biochemically, as undiagnosed pheochromocytoma can cause life-threatening hypertensive crisis. 1, 2
Alternative Imaging: Chemical Shift MRI
Chemical shift MRI can be used as an alternative second-line imaging modality for indeterminate masses, though it has similar limitations with heterogeneous signal intensity drop being seen in some pheochromocytomas 1, 2