“An indeterminate adrenal lesion that enhances greater than 130 HU on multidetector CT cannot be assumed to be an adenoma. Hypervascular pheochromocytoma (>130 HU) mimics adenoma washout pattern; absolute venous phase enhancement level must be considered.”
Adrenal Adenoma and Pheochromocytoma: Comparison of Multidetector CT Venous Enhancement Levels and Washout Characteristics. 
Northcutt BG, Trakhtenbroit MA, Gomez EN, Fishman EK, Johnson PT.
J Comput Assist Tomogr. 2016 Mar-Apr;40(2):194-200.

Objective: To evaluate the proportion of pheochromocytomas meeting the criteria for adenoma on adrenal washout CT and the diagnostic performance of adrenal washout CT for differentiating adenoma from pheochromocytoma.
Conclusions: There was a non-negligible proportion of pheochromocytomas meeting the criteria for adenoma on adrenal washout CT. Although overall diagnostic performance was excellent for differentiating adenoma from pheochromocytoma, specificity was relatively low.
Pheochromocytoma as a frequent false-positive in adrenal washout CT: A systematic review and meta-analysis.
Woo S, Suh CH, Kim SY, Cho JY, Kim SH. 
Eur Radiol. 2018 Mar;28(3):1027-1036.

Key points: 
• Non-negligible proportion of pheochromocytomas can be mistaken for adenoma. 
• Adrenal washout CT showed good sensitivity (97%) but relatively low specificity (67%). 
• Findings other than washout percentage should be used when diagnosing pheochromocytomas.
Pheochromocytoma as a frequent false-positive in adrenal washout CT: A systematic review and meta-analysis.
Woo S, Suh CH, Kim SY, Cho JY, Kim SH. 
Eur Radiol. 2018 Mar;28(3):1027-1036.

“ Pheochromocytomas are often considered the great mimicker of other adrenal tumors. Because of their varied clinical, imaging, and pathologic appearances, accurate diagnosis can be challenging.”
Pheochromocytoma: The Range of Appearances on Ultrasound, CT, MRI, and Functional Imaging
Leung K et al.
AJR 2013; 200:370-378

“Cystic appearance was present in 12/22 (55%, 95% CI 32-76%) pheochromocytomas (mean size 5.3 cm), 15/34 (44%, 95% CI 27-62%) malignant masses (mean size 5.8 cm), and 2/36 (5.6%, 95% CI 0.7-9%) adenomas (mean size 3.2 cm). Sensitivity and specificity of cystic appearance for distinguishing pheochromocytoma or malignant masses from adenomas were 48.2% (95% CI 34.7-62.0%) and 94.4% (95% CI 81.3-99.3%), respectively. Cystic appearance was a significant predictor of tumor type (p = 0.015) even after controlling for tumor size. Reader agreement for cystic appearance was almost perfect with a kappa of 0.85.”
Accuracy of focal cystic appearance within adrenal nodules on contrast-enhanced CT to distinguish pheochromocytoma and malignant adrenal tumors from adenomas. 
Corwin MT et al. 
Abdom Radiol (NY). 2021 Jan 8. doi: 10.1007/s00261-020-02925-5.