What type of computed tomography (CT) scan is recommended to evaluate the adrenal glands in a patient with hypercortisolism and low adrenocorticotropic hormone (ACTH) levels?

CT Adrenal Protocol for ACTH-Independent Cushing's Syndrome

Order an adrenal protocol CT scan that includes both unenhanced (non-contrast) imaging and contrast-enhanced imaging with delayed washout at 15 minutes to evaluate for adrenal adenoma versus carcinoma in this patient with elevated cortisol and low ACTH. 1

Initial Imaging Approach

The combination of elevated cortisol with low ACTH indicates ACTH-independent Cushing's syndrome, which localizes the pathology to the adrenal glands themselves (either benign adenoma, malignant carcinoma, or rarely bilateral hyperplasia). 1 This biochemical pattern excludes pituitary or ectopic ACTH-secreting tumors, making adrenal imaging the definitive next step. 1

Specific CT Protocol Requirements

The adrenal protocol CT must include these specific components to distinguish benign from malignant lesions:

Unenhanced (Non-Contrast) Phase

• Measure Hounsfield units (HU) on the unenhanced scan—values ≤10 HU strongly suggest benign adenoma, while values >10 HU raise concern for malignancy. 1
• The unenhanced phase is critical because it provides the baseline attenuation needed to calculate contrast washout. 1

Contrast-Enhanced Phase with Delayed Washout

• If the HU attenuation is >10 on unenhanced CT, proceed with IV contrast administration followed by delayed imaging at 15 minutes to calculate enhancement washout. 1
• Enhancement washout >60% at 15 minutes indicates a benign lesion, while washout <60% suggests possible malignancy. 1

Additional Imaging Characteristics to Assess

• Tumor size: Malignancy should be suspected if the tumor is >5 cm. 1
• Morphology: Look for irregular margins, internal heterogeneity, local invasion, or adjacent lymphadenopathy—all concerning for adrenal carcinoma. 1
• Evaluate for metastases: The scan should include chest, abdomen, and pelvis to assess for metastatic disease if malignancy is suspected. 1

Alternative: Chemical-Shift MRI

If CT findings remain indeterminate or if there are contraindications to IV contrast, chemical-shift MRI is highly sensitive and specific for differentiating benign from malignant adrenal tumors. 1 Most benign adenomas contain intracellular lipid and show characteristic signal intensity drop on out-of-phase images, while malignant tumors typically do not contain lipid. 1, 2 MRI also better delineates local invasion and inferior vena cava involvement than CT. 1

Critical Clinical Pitfalls

• Never perform adrenal biopsy before excluding pheochromocytoma biochemically—this patient needs screening for catecholamine excess before any invasive procedure. 1
• Biopsy of suspected adrenal carcinoma should be avoided due to risk of tumor seeding along the needle tract. 1
• Be aware that approximately one-third of adrenal adenomas do not washout in the typical benign range, and some malignant masses can show washout patterns mimicking adenomas. 1
• Pheochromocytomas can also demonstrate washout characteristics similar to adenomas in roughly one-third of cases, creating potential diagnostic confusion. 1

Management Based on Imaging Results

If Benign Adenoma Confirmed (≤10 HU or >60% washout)

• Proceed with laparoscopic adrenalectomy when feasible for definitive treatment. 1
• Plan for postoperative corticosteroid supplementation until recovery of the hypothalamus-pituitary-adrenal axis, as the contralateral gland will be suppressed. 1

If Malignancy Suspected (>5 cm, irregular margins, heterogeneous, <60% washout)

• Open adrenalectomy is preferred over laparoscopic approach due to increased risk of local recurrence and peritoneal spread with minimally invasive surgery. 1
• Surgical resection should include removal of adjacent lymph nodes and may require en bloc resection of adjacent structures (liver, kidney, pancreas, spleen, diaphragm) for complete tumor removal. 1

If Bilateral Abnormalities Detected

• Consider adrenal vein sampling to measure cortisol production from each gland to determine if unilateral or bilateral adrenalectomy is needed. 1
• If cortisol production is asymmetric, perform laparoscopic unilateral adrenalectomy of the most active side; if symmetric, medical management is indicated. 1