Utility of the 10 Hounsfield unit threshold for identifying adrenal adenomas: Can we improve?

Background: Current recommendations using Hounsfield units (HU) ≤ 10 to identify adrenal adenomas on unenhanced computed tomography (CT) miss 10-40% of benign adenomas. We sought to determine if changing HU threshold and adding absolute percent contrast washout (APW) criteria would identify adrenal adenomas better than current recommendations.

Methods: Imaging characteristics were compared between patients with adenomas (n = 128) and those with non-adenomas (n = 54) after unilateral adrenalectomy. Sensitivity, specificity, positive and negative predictive values (PPV, NPV) were calculated.

Results: Using HU ≤ 10 to identify adenomas had a sensitivity of 47.6%, specificity of 93.3% (AUC = 0.71, p < 0.001), PPV of 95.3%, and NPV of 58.1% for identifying adrenal adenomas. Applying HU ≤ 16 improved sensitivity (65.4%) without reducing specificity (93.3%) (AUC = 0.79, p < 0.001), PPV increased to 96.3%, and NPV decreased to 47.6%. Applying HU ≤ 16 as the initial criterion followed by APW > 60% for lesions exceeding 16 HU, sensitivity increased to 93.4%, specificity was 93.3% and PPV 96.6%, and NPV improved to 85.7% (AUC = 0.96, p < 0.001).

Conclusions: Criteria of initial threshold of HU ≤ 16 followed by APW > 60% for lesions exceeding 16 HU yielded improved sensitivity and specificity in identification of adrenal adenomas.