What is the initial imaging test to evaluate for adrenal adenomas after an abnormal low-dose Dexamethasone (corticosteroid) suppression test with low plasma Adrenocorticotropic hormone (ACTH)?

From the Guidelines

The initial imaging test to evaluate for adrenal adenomas in a patient with an abnormal low-dose dexamethasone suppression test and low plasma ACTH is a non-contrast CT scan, as recommended by the most recent guidelines 1. This approach is supported by the 2023 guideline from the Journal of Urology, which suggests that patients with an indeterminate incidental adrenal mass should undergo a non-contrast CT as the first-line imaging to distinguish benign lesions from those requiring further radiologic investigation. The non-contrast CT scan provides valuable information about the density of the adrenal mass, with benign adenomas typically demonstrating low attenuation (<10 Hounsfield units) 1. If the non-contrast CT scan is inconclusive, further imaging with either washout CT or chemical-shift MRI may be necessary to characterize the adrenal mass 1. It's essential to note that the choice of imaging modality should be guided by the clinical context and the presence of any contraindications to certain tests. In general, a non-contrast CT scan is a reasonable initial imaging test for evaluating adrenal adenomas in patients with abnormal biochemical test results, such as an abnormal low-dose dexamethasone suppression test and low plasma ACTH. Some studies have suggested that contrast-enhanced CT scans or MRI may also be useful in certain situations, but the most recent and highest-quality evidence supports the use of non-contrast CT as the initial imaging test 1. Key points to consider when evaluating adrenal adenomas include:

• The importance of using non-contrast CT as the initial imaging test
• The value of assessing the density of the adrenal mass on non-contrast CT
• The potential need for further imaging with washout CT or chemical-shift MRI if the non-contrast CT is inconclusive
• The importance of considering the clinical context and any contraindications to certain tests when choosing an imaging modality.

From the Research

Initial Imaging Test for Adrenal Adenomas

The initial imaging test to evaluate for adrenal adenomas after an abnormal low-dose dexamethasone test is abnormal with low plasma ACTH can be determined based on the following evidence:

• Computed Tomography (CT) is a commonly used initial imaging modality for evaluating adrenal incidentalomas, with a sensitivity of 0.71 and specificity of 1.0 2.
• Magnetic Resonance Imaging (MRI) can be used in addition to CT to further characterize adrenal lesions, with a sensitivity of 0.86 and specificity of 1.0 when combined with CT 2.
• CT scan is also useful in detecting and characterizing adrenal adenomas, especially when combined with unenhanced and dynamic scans 3.
• Other imaging modalities such as positron emission tomography (PET) using tracers like 11C-metomidate or 18F-fluoro-2-deoxyglucose (FDG) can provide additional information, but are not typically used as the initial imaging test 2, 4, 3.

Comparison of Imaging Modalities

The following points compare the different imaging modalities:

• CT scan has a higher sensitivity and specificity than MRI in detecting adrenal adenomas 2, 3.
• MRI can be useful in identifying lipid-rich adenomas, which can be characterized by a loss of signal intensity on out-of-phase breath-hold gradient-echo images 3.
• PET/CT can provide functional information about adrenal lesions, but is not typically used as the initial imaging test 4.
• Radiocholesterol scintigraphy, such as dexamethasone-suppression norcholesterol scintigraphy (NCS), can be useful in evaluating primary aldosteronism, with a high positive predictive value (PPV) of 97.6% 5.