CT characteristics of pheochromocytoma - Relevance for the evaluation of adrenal incidentaloma

Background: Up to 7% of all adrenal incidentalomas (AIs) are pheochromocytomas (PCCs). In the evaluation of AI, it is generally recommended to exclude PCC by measurement plasma free or 24h urinary fractionated metanephrines. However, recent studies suggest to abstain from biochemical exclusion of PCC in cases of lesions with computed tomography (CT) characteristics of an adrenocortical adenoma (ACA).Aim: To determine the proportion of PCCs with ACA-like attenuation or contrast washout on CT.Methods: For this multicenter retrospective study, two central investigators independently analyzed the CT reports of 533 patients with 548 histologically confirmed PCCs. Data on tumor size, unenhanced Hounsfield Units (HU), absolute percentage washout (APW) and relative percentage washout (RPW) were collected besides clinical parameters.Results: Among the 376 PCCs for which unenhanced attenuation data were available, 374 had an attenuation of >10 HU (99.5%). In the two exceptions (0,5%), unenhanced attenuation was exactly 10 HU, which lies just within the range of ≤10 HU that would suggest a diagnosis of ACA. Of 76 PCCs with unenhanced HU >10 and available washout data, 22 (28,9%) had a high APW and/or RPW, suggestive of ACA.Conclusion: Based on the lack of PCCs with an unenhanced attenuation of <10 HU, and the low proportion (0,5%) of PCCs with an attenuation of =10 HU, it seems reasonable to abstain from biochemical testing for PCC in AIs with an unenhanced attenuation ≤10 HU. The assessment of contrast washout, however, is unreliable to rule out PCC.