What are the imaging washout characteristics of pheochromocytoma?

From the Guidelines

Pheochromocytomas are best distinguished from adrenal adenomas by their delayed washout on contrast-enhanced CT imaging, with washout values typically less than 60% at 15 minutes, as opposed to adrenal adenomas which show rapid washout of contrast (>60% absolute washout at 15 minutes) 1.

Key Imaging Characteristics

• Pheochromocytomas characteristically appear bright on T2-weighted MRI images due to their high fluid content, known as the "light bulb" sign.
• The gold standard for functional imaging of pheochromocytomas is 123I-MIBG (metaiodobenzylguanidine) scintigraphy, which has high specificity as it is taken up by catecholamine-producing cells.
• PET imaging with 18F-FDG or more specific tracers like 18F-FDOPA and 68Ga-DOTATATE can also be valuable, particularly for metastatic disease.

Limitations of Washout CT

• Roughly 1/3 of pheochromocytomas may washout in the characteristic range of an adenoma.
• Approximately 1/3 of adrenal adenomas do not washout in the adenoma range.
• Malignant masses can also washout in the adenoma range, which can result in adrenal cortical carcinoma or hyper-vascular metastases being mistaken for an adenoma on a CT washout.

Alternative Imaging Options

• Chemical shift MRI can be used as an alternative to washout CT, exploiting the different frequency of protons in water and fat to detect microscopic fat.
• When microscopic fat is identified as a homogeneous signal intensity drop on MRI, these features are diagnostic of lipid-rich adenoma.
• Heterogeneous signal intensity drop is a more controversial imaging finding since minute amounts of microscopic fat have been identified in pheochromocytoma, adrenal cortical carcinoma, and some metastases 1.

From the Research

Pheochromocytoma Imaging Washout Characteristics

• Pheochromocytomas can have variable imaging appearances, including low attenuation on unenhanced CT and high percentage of contrast washout on delayed scanning, which can mimic adrenal adenomas 2, 3.
• The washout pattern of pheochromocytomas can be inconsistent, with some studies showing a high percentage of contrast washout, while others demonstrate a more variable washout profile 2, 4.
• A systematic review and meta-analysis found that 35% of pheochromocytomas met the criteria for adenomas on adrenal washout CT, highlighting the potential for false positives 4.
• The diagnostic performance of adrenal washout CT for differentiating adenoma from pheochromocytoma showed good sensitivity (97%) but relatively low specificity (67%) 4.
• Other imaging modalities, such as positron emission tomography, have been shown to be effective in localizing pheochromocytomas, particularly in cases where conventional imaging modalities are inconclusive 5.

Key Findings

• Pheochromocytomas can exhibit a range of imaging appearances, making diagnosis challenging 3, 6.
• Adrenal washout CT can be used to characterize pheochromocytomas, but its specificity is relatively low 4.
• A combination of imaging modalities and clinical correlation is necessary for accurate diagnosis of pheochromocytomas 2, 3, 6, 4, 5.