How do I determine the likelihood of malignancy in an adrenal incidentaloma based on imaging characteristics?

To determine the likelihood of malignancy in an adrenal incidentaloma based on imaging characteristics, the primary factors to assess include lesion size, imaging density on non-contrast CT, and contrast washout patterns. Lesions smaller than 4 cm with homogeneous, low attenuation (<10 Hounsfield units) on non-contrast CT are highly likely to be benign adenomas, whereas lesions larger than 4 cm or with higher attenuation values raise suspicion for malignancy or pheochromocytoma NICE NG99 Singh & Buch 2008. A non-contrast CT attenuation value greater than 10 HU suggests a lipid-poor lesion, which is more likely to be malignant or a metastasis Aron et al. 2012. Additionally, contrast-enhanced CT with delayed imaging can be used to calculate absolute and relative percentage washout; benign adenomas typically show rapid contrast washout (>50% absolute washout at 10-15 minutes), while malignant lesions demonstrate slower washout NICE NG99 Paschou et al. 2016. Irregular margins, heterogeneity, necrosis, and evidence of local invasion on imaging further increase the likelihood of malignancy NICE NG99 Aron et al. 2012. Magnetic resonance imaging (MRI) with chemical shift imaging can also help differentiate adenomas from malignant lesions by detecting intracellular lipid content, with adenomas showing signal loss on out-of-phase images Singh & Buch 2008. In summary, integrating lesion size, non-contrast CT attenuation, contrast washout characteristics, and morphological features on imaging provides the best approach to estimate malignancy risk in adrenal incidentalomas NICE NG99 Paschou et al. 2016.