Although uncommon, various adrenal masses can mimic adrenal adenomas, mainly because of low attenuation on CT or signal loss on out-of-phase compared to in-phase MR pulse sequences. Simple cyst with an attenuation value of less than 10 HU can mimic adrenal lipid-rich adenoma on unenhanced CT. However, they do not enhance on postcontrast series (Fig. 19.17) and exhibit markedly increased signal intensity on T2-weighted MR images (Fig. 19.18). Metastatic deposits containing intracellular lipid can develop secondary to primary malignancies containing intracellular lipid, such as hepatocellular carcinoma (HCC) or renal cell carcinoma (RCC-clear cell subtype) (Fig. 19.19) [13, 14]. The presence of intracellular lipid in these masses results in signal loss on out-of-phase compared with in-phase pulse sequences, making it difficult to differentiate them from the more common adrenal adenoma. Adrenal cortical carcinoma (ACC) has been also reported to contain intracellular lipid [15]. It should be noted, however, that ACCs are usually large at presentation, and the distribution of intracellular lipid in ACC is rather inhomogeneous (Fig. 19.20).

The most common macroscopic fat-containing adrenal mass is myelolipoma. Myelolipoma is an uncommon benign tumor composed of mature adipose tissue and hematopoietic tissue. Most of these lesions are discovered incidentally. The fatty component of this tumor can be diagnosed by the presence of areas of negative attenuation value on CT (Fig. 19.25). On MRI, macroscopic fat is hyperintense on non-fat-suppressed, T1-weighted images. The use of fat suppression can help confirm the diagnosis by demonstrating a loss of signal intensity in the fatty component (Fig. 19.26) [19]. Myelolipomas can be large and symptomatic, secondary to spontaneous hemorrhage (Fig. 19.27). Rarely, large myelolipomas are confused with other retroperitoneal lipomatous tumors, such as liposarcoma (Fig. 19.28) [11].

Congenital adrenal hyperplasia can have a characteristic appearance of multiple bilateral adrenal masses containing extensive macroscopic fat, which may result from prolonged stimulation of the adrenal cortex by elevated ACT levels (Fig. 19.29).

We have described a rare entity presumed to represent lipomatous metaplasia [20]. Adrenal lipomatous metaplasia is a known pathological entity characterized by the presence of small oval foci of macroscopic lipid occupying an otherwise unremarkable adrenal cortex (Fig. 19.30). Because all the cases have been reported in the pathology literature, it is not surprising that they have been found in patients with hypersecretory adrenal lesions such as hyperplasia, adenoma, and carcinoma, which required surgical resection. In our cases, we found no clinical evidence of hypersecretory or structural adrenal abnormalities [20]. ACC has been rarely reported to contain foci of macroscopic fat, considering the amount of macroscopic fat in the lesion. However, all the described cases of macroscopic fat-containing ACC have contained only a small amount of fat compared with the overall lesion size; thus, this may not be a reliable distinguishing feature because it is well known that myelolipomas contain widely varying amounts of fat, with some containing very little fatty tissue. For this reason, we feel that it is important to consider additional features of the lesion, including the presence of hypertension or clinical or biochemical stigmata of adrenal hormonal excess, which favor the presence of ACC rather than myelolipoma, and the amount of enhancement after intravenous contrast administration. Although myelolipomas can demonstrate enhancement, the presence of a large heterogeneous mass with significant peripheral enhancement more likely represents ACC than myelolipoma (Fig. 19.31) [21].