Imaging characteristics

Ultrasound provides inadequate visualization of adrenal pathology, especially of the left adrenal gland.7 As such, adrenals are best visualized with cross-sectional imaging studies. Whether employing magnetic resonance (MR) or computed tomography (CT), assessment of intracytoplasmic lipid content forms the basis of adrenal imaging, since high cellular lipid is pathognomonic for adrenal adenoma—the most common adrenal lesion.8 As such, low attenuation (less than 10 HU) on non-contrast CT scan is diagnostic for adrenal adenoma.9,10

Similarly, opposed phase chemical-shift MR imaging, as qualified by signal drop out, can prove the presence of intra­-
cellular lipid and thus confirm the presence of adrenal adenoma.10,11 Macroscopic lipid is diagnostic of myelolipoma.1 It is important to understand that non-contrast CT and MR imaging (MRI) are largely equally informative with regard to assessment of intracellular lipid. Nevertheless, 30% of adrenal adenomas are lipid-poor, also termed “atypical” by some authors, and cannot be differentiated from non-
adenomas on non-contrast CT nor MRI.12

Instead, the vast majority of these lipid-poor adenomas can be characterized employing a CT washout study. This imaging methodology quantitates loss of contrast enhancement by a lesion and thus informs of lipid content in a given mass.8,11,13 As such, adenomas, including the atypical lipid-poor adenomas, demonstrate contrast washout, while adrenocortical carcinomas, pheochromocytomas, and metastases to the adrenal do not “wash out” on these studies.1,14

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An online calculator also can be found at CT washout studies demonstrate excellent test characteristics,11 but not all adenomas exhibit washout and rare adrenocortical carcinomas and pheochromocytomas have been known to demonstrate lipid-rich behavior.15,16

Importantly, standard post-contrast CT (generally obtained one minute following an iodinated contrast bolus), unlike non-contrast CT, does not provide meaningful information regarding the lipid content of adrenal masses.17 Furthermore, washout studies cannot be obtained with MRI because gadolinium-based contrast agents do not possess dose-dependent signal fall-off properties that are seen for iodinated CT agents.18

Tumor size

On average, incidental adrenal masses are approximately 3 cm in diameter at the time of diagnosis.19 Meanwhile, adrenocortical carcinomas, present at a mean diameter of 11 cm.5 As such, in retrospective analyses of patient cohorts with adrenal incidentaloma, size has had a strong association with malignancy risk.7,19,20 Interestingly, as adrenal tumors grow large, the kidney is often displaced (Figure 1). Generally, this is in contrast to similarly sized tumors that stem from the upper pole of the kidney where the renal unit remains in its orthotopic location.1


The association of malignancy with tumor size at presentation has resulted in the proposal of size thresholds that necessitate resection. Generally, masses larger than 6 cm (that are not consistent with myelolipoma on imaging) are resected, given a greater than 30% risk of malignancy in retrospective series.6,7 Limited evidence exists regarding how to reconcile worrisome tumor size with reassuring imaging characteristics. Isolated case reports reveal that rare adrenocortical carcinomas can exhibit lipid-rich behavior, and thus the presence of washout on adrenal protocol CT does not completely rule out malignancy in large lesions.15,21

Meanwhile, tumors smaller than 4 cm should largely be observed unless metabolically active.1,6 Controversy regarding management of 4-6 cm adrenal lesions exists, and decisions should hinge on clinical judgment as informed by age at diagnosis, patient comorbidities, and tumor growth kinetics.1,4,6,19,22,23

Tumor growth 

Current recommendations suggest imaging at 6, 12, and 24 months following identification of adrenal mass to establish tumor growth rates.24 Certainly, worrisome adrenal masses should be imaged earlier, whereas small masses in elderly/comorbid patients in whom intervention is unlikely to be justified do not deserve such intense follow-up.1

Some authors have suggested that all masses that exhibit greater than 1 cm growth should be excised.6 In fact, 5%-9% of adrenal incidentalomas exhibit this behavior at two to three years of follow up, while malignancy risks are estimated to be 1 in 1,000 in such cases.7, 25 As such, patients who are taken for resection due to tumor growth must be counseled appropriately.1

Adrenal biopsy

Although adrenal biopsy is associated with a low risk of complications, needle-track seeding by adrenocortical carcinoma and otherwise avoidable challenges during laparoscopic adrenalectomy following biopsy have been reported.26-28

Importantly, adrenal biopsy is unable to differentiate between adrenocortical carcinoma and benign adenoma and, as such, rarely yields clinical information that results in a change in management.27 If a biopsy is deemed necessary (e.g., in a patient with a primary malignancy where documentation of metastatic disease will alter management), metabolic work-up to exclude pheochromocytoma is obligatory.1