Adrenal adenomas are the most common adrenal mass lesion and are often found incidentally during abdominal imaging for other reasons. In all cases, but especially in the setting of known current or previous malignancy, adrenal adenomas need to be distinguished from adrenal metastases or other adrenal malignancies.
The term incidental adrenal lesion (also colloquially known as an incidentaloma) is sometimes used interchangeably with adrenal adenoma, although in truth an incidental adrenal lesion includes all pathologies (including malignancies). As such, the term should be avoided lest it results in confusion.
Adrenal adenomas are found in almost all age groups but increase in frequency with age 4.
The majority (~95%) of adrenal adenomas are non-functioning, in which case they are asymptomatic.
Patients with hyperfunctioning adrenal gland adenomas present with manifestations of excess hormone secretion. The most common disease states caused by functioning adenomas are Cushing syndrome (due to excess cortisol production), Conn syndrome (due to excess aldosterone production) or sex hormone-related symptoms 4.
Imaging plays a key role in assessing the vast number of incidental adrenal lesions, the majority of which are adrenal adenomas. Correlation with previous imaging is often useful, as a lesion which has not changed over some years is unlikely to be malignant.
They can be divided into those that have typical or atypical appearances.
Typical adenomas are:
- small: <3 cm
- homogeneous and low density
Atypical features include:
- no fat
CT is often the modality which identifies an adrenal mass. Fortunately using density is highly sensitive and specific as 70% of adrenal adenomas contain significant intracellular fat. Lipid-poor adenomas are more difficult to diagnose because the CT density increases and approach those of soft tissue.
For lipid-poor lesions, the contrast washout rate can be calculated at CT. Adenomas typically have rapid contrast washout, whereas non-adenomas tend to wash out more slowly. There are different protocols, and some controversy exists as to which protocol is the best. A 5 or 10-minute protocol may be more suitable for busy CT lists. However, there is evidence that a 15 minutes post contrast protocol has better diagnostic accuracy.11
- non-contrast imaging 4
- <0 HU: considered 47% sensitive and 100% specific
- <10 HU: considered 71% sensitive and 98% specific
- washout imaging
It is important to note that hypervascular metastases may show identical washout values, particularly those from renal cell carcinoma and hepatocellular carcinoma. An alternative diagnosis to adrenal adenoma must be considered when there is a value >120 HU on the portal venous phase, particularly in cases with a prior history of neoplasm 12.
Chemical shift imaging is the most reliable for diagnosis especially when CT findings are equivocal. Because of the high sensitivity of chemical shift MR imaging to minute amounts of intravoxel fat, MR imaging demonstrates signal intensity loss on opposed-phase images in the majority of adenomas, and a drop in signal intensity of greater than 20% is considered diagnostic for an adenoma 2. Rather than measuring the signal, one can compare the adenoma in and out of phase, with images windowed similarly (using the spleen or muscle as a reference - NB do not use the liver as it can change signal on in and out of phase imaging depending on the presence of haemochromatosis or hepatic steatosis) 4.
As MRIs are usually performed to help indeterminate CT lesions, the sensitivity and specificity depend on the CT density. MRI is useful in adrenal mass with an attenuation <30 HU. A drop in signal on out-of-phase imaging for:
- 10-30 HU on CT is 89% sensitive and 100% specific
- 10-20 HU on CT is 100% sensitive and 100% specific
Malignant adrenal lesions also demonstrate restricted diffusion 4.
Treatment and prognosis
Small adrenal mass with manifestations of hormonal excess need resection, as do large (>3-5 cm) non-functioning adrenal mass lesions as they are considered potentially malignant (see adrenal carcinoma).
Small adrenal lesions with typical features of adenomas and without biochemical abnormality can be safely left in situ. Some studies demonstrate up to 40% may grow and approximately 10% have been shown to resolve 14.
In patients with a known malignancy, ~50% of nonspecific adrenal nodules will represent adrenal adenomas.
Consider other adrenal lesions such as:
- adrenocortical carcinoma
- adrenal metastasis
- focal adrenal granulomatous disease
- adrenal myelolipoma
small (<3 cm), homogeneous, and low density (<10HU) are leave alone lesions
if >120HU on the portal venous phase, the washout should be ignored in determining a lipid-poor adenoma, most likely the lesion is a hypervascular metastasis or pheochromocytoma
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