The adrenal gland is a common site of metastatic disease, with a reported prevalence of 3.1% in one large retrospective study.  Modern cross-sectional imaging, such as computed tomography (CT) scanning and magnetic resonance imaging (MRI), has revealed that adrenal metastases occur more frequently than previously believed. CT scans of normal right and left adrenal glands are shown in the images below.
A diagnosis of adrenal metastasis is important in examining patients with cancer, because metastasis indicates stage IV disease. (In this article, references to staging refer to tumor, nodes, and metastases [TNM] staging, unless otherwise stated.) Except in ipsilateral renal cancer, the presence of metastases almost always influences the choice of treatment.
Benign adrenal tumors may occur in the general population and in patients with cancer. In the general population, it has been estimated that 2-3% of incidentally found adrenal masses are malignant. 
Noninvasive characterization is important, because it prevents unnecessary biopsy. In adults, CT scanning is the ideal imaging modality for detecting abnormal adrenal glands (see the images below). In patients with primary malignancies that have known predilections for adrenal metastases (especially melanoma, lung cancer, and breast cancer), CT scanning of the abdomen is useful for staging to exclude metastatic disease. 
The usefulness of ultrasonography is limited to detecting large tumors in adults. Children, in whom imaging is less difficult, may be suitable candidates for US.
Often, MRI is expensive and not widely available. MRI is useful for further noninvasive characterization of adrenal masses.
Radionuclide studies may help to demonstrate that a lesion is a functioning adenoma, although false-negative results are known.
Positron emission tomography (PET) scanning may be helpful in differentiating incidental adrenal adenomas (the lipid-poor variety) from small metastases.
A finding of a focal mass in the adrenal glands does not necessarily indicate metastasis. Percutaneous biopsy often is the next step if CT and MRI scan findings do not help in conclusively diagnosing benign conditions.
Limitation of techniques
Imaging findings can demonstrate metastasis only if an alteration occurs in the outline, the size, or the internal characteristics of the adrenal gland. A normal-appearing gland does not exclude microscopic involvement.
One study of patients with small-cell lung cancer showed that as many as 17% of adrenal glands with morphologically normal CT scan findings had positive results for metastasis on fine-needle aspiration. Small adrenal metastases (< 2 cm) are difficult to detect with ultrasonography.
Plain-film imaging of adrenal masses is limited. Large masses are often indistinguishable from renal lesions, and the presence of calcium is not a specific finding (see the image below). The presence of phleboliths is suggestive of an adrenal hemangioma.
If the plain-film radiographic findings suggest an adrenal lesion, further evaluation may be performed with CT scanning or MRI. The choice of investigation is determined by availability.
The negative predictive value of normal abdominal radiographic findings is low, because even images of relatively large adrenal lesions may not demonstrate changes.
Adrenal metastases appear as focal masses (see the image below) or distortion of the contour of the adrenal gland. Larger lesions may have central necrosis or hemorrhage. These lesions are heterogeneous and may have thick, enhancing rims. They may also invade contiguous organs, such as the kidneys. Adrenal metastases of less than 3 cm may be homogeneous. [4, 5]
Attenuation values of less than 10 HU on unenhanced CT scans of the adrenal glands are indicative of lipid-containing benign lesions, such as adrenal adenomas (specificity, 100%; sensitivity, 85%); however, lipid-poor adrenal adenomas have attenuation values of more than 10 HU. Attenuation values of 24-37 HU on enhanced CT scans obtained 15 minutes to 1 hour after the administration of contrast material are also indicative of benign lesions (specificity, 96%; sensitivity, 79%). Washout imaging is critical for the characterization of lipid-poor adrenal adenomas. 
The attenuation values of the adrenal glands are more useful than their size for making the diagnosis. The use of the size threshold alone has poor specificity regarding high thresholds (size >2.5 cm: specificity, 79%; sensitivity, 84%) and poor sensitivity regarding low thresholds (size < 1.5 cm: specificity, 93%; sensitivity, 16%).  Bilateral involvement may be seen in a number of benign conditions, such as adrenal adenomas, pheochromocytomas, and tuberculosis.
CT scan findings that may mimic those of left adrenal masses include a mass in the upper pole of the left kidney, gastric diverticulum, splenic lobulation, and a large mass in the tail of the pancreas.
The image below demonstrates CT scan findings that confirm the absence of a mass.
Magnetic Resonance Imaging
Adrenal metastases are usually hypointense on T1-weighted images and relatively hyperintense on T2-weighted images (see the first image below). The exception is metastatic melanoma, which may be bright on T1-weighted images (see the second image below). Occasionally, lesions may remain hyperintense on long–echo-time, T2-weighted images, mimicking pheochromocytomas.
Chemical-shift imaging with in-phase and out-of-phase imaging techniques is used to exclude metastatic disease by detecting the presence of intracellular lipid within adrenal gland lesions. On out-of-phase images, the lesions have lower signal intensity, because the signal from lipid cancels the signal from water by an amount that varies (depending on the quantity of fat present). 
By comparing out-of-phase images with in-phase images (in which signals from lipid and water are summed), very small amounts of lipid that cannot be demonstrated with other methods can be detected; however, lipid-poor adenomas do not lose signal intensity on out-of-phase images. An adrenal metastasis might not contain lipid (see the image below).
Conventional spin-echo MRI and contrast-enhanced MRI findings with benign and malignant conditions have too much overlap to be useful. A small number of malignant tumors, such as metastatic hepatocellular carcinoma, metastatic renal cell carcinoma, metastatic liposarcoma, and adrenocortical carcinoma, may contain enough fat to decrease the signal intensity on out-of-phase images.
Adrenal metastases appear as solid lesions with heterogeneous echogenicity on sonograms, as seen in the image below. The echogenicity of the lesions is usually less than that of the surrounding fat (hypoechoic).
Ultrasonography of the adrenal glands in adults is technically difficult to perform. When it is performed by an experienced operator, ultrasonography can reveal the right adrenal gland in 92% of patients and the left adrenal gland in 71% of patients. Scanning is performed after the patient fasts to reduce bowel gas. In newborns, the adrenal glands are easily imaged. The medulla of the adrenal gland is highly echogenic, and the adrenal cortex is less echogenic. The adrenal glands are more easily visualized in typically sized adults (those with a thin habitus). [7, 8]
False-positive and false-negative findings may include the following:
Thickened diaphragmatic crus
Retrocrural and retroperitoneal adenopathy
Upper-pole renal cyst and/or neoplasm
Hypertrophic caudate lobe
Nuclear scintigraphy with iodomethylnorcholesterol (NP-59) may be helpful in differentiating benign from malignant lesions. Gross et al showed scintigraphy with NP-59 was 71% sensitive, 100% specific, and 93% accurate when determining whether an adrenal lesion was benign or malignant.  Benign lesions show uptake, although hemorrhage and inflammatory masses are confounding factors.
Imaging with fluorodeoxyglucose (FDG) is an alternative technique that is performed with positron emission tomography (PET). Various studies have shown evidence of success with FDG PET scanning. [10, 11, 12, 13, 14, 15] . PET has been shown to be excellent at differentiating adrenal lesions detected by CT or MRI, with 93-100% sensitivity, 90-94% specificity, and 92-96% accuracy.  PET-CT may lead to false-negative findings with lesions that are less than 10 mm in diameter or in the setting of hemorrhage or necrosis. PET-CT adrenal lesions are considered positive when the maximum standardized uptake value (SUVmax) is greater than that of the liver, although SUV greater than 3.1 has also been suggested.  (See the image below.)