How can I differentiate between benign adrenal adenomas and adrenocortical carcinoma based on imaging findings?

To differentiate between benign adrenal adenomas and adrenocortical carcinoma (ACC) on imaging, key features on CT and MRI are used. Benign adrenal adenomas typically present as small (<4 cm), homogeneous lesions with smooth margins and low attenuation on unenhanced CT (<10 Hounsfield units), reflecting their high lipid content. They also show rapid contrast washout (>50% absolute washout at 10-15 minutes) on delayed contrast-enhanced CT, which is characteristic of adenomas. In contrast, ACCs are usually larger (>4 cm), heterogeneous with irregular or lobulated margins, and have higher unenhanced CT attenuation values (>10 HU) due to lower lipid content. They often demonstrate delayed contrast washout (<40%) and may show necrosis, calcifications, or local invasion on imaging. MRI can further aid differentiation: adenomas typically have signal loss on out-of-phase sequences due to intracellular lipid, whereas ACCs lack this feature and may show heterogeneous enhancement and restricted diffusion. These imaging criteria are supported by UK guidelines emphasizing size, attenuation, and washout characteristics as primary discriminators ¹, while recent literature (Low et al., 2012; Oguro et al., 2025; Grazzini et al., 2025) reinforces the importance of combining multiple imaging features including morphology, enhancement patterns, and functional imaging to improve diagnostic accuracy.