M. Kohli1, R. Randle1, S. Pitt1, D. Schneider1, R. Sippel1 1University Of Wisconsin,Department Of Surgery,Madison, WI, USA
Introduction: Adrenal incidentalomas are identified on up to 5% of abdominal CT scans. Assessing such lesions for malignancy is essential for establishing appropriate patient follow up. A threshold of 10 Hounsfield units (HU) is currently recommended for differentiating benign adenomas from non-adenomas. Our study aims to evaluate the utility of the 10 HU threshold and to determine whether additional CT imaging features can be used to identify adenomas.
Methods: We performed a retrospective review of a single institution’s prospective endocrine surgery database. Our cohort included 192 patients who underwent an adrenalectomy between 2001 and 2015 due to a unilateral adrenal mass (excluding pheochromocytoma). All masses that were non-adenomatous via surgical histology (adrenal cortical carcinomas, ganglioneuromas, metastases, etc.) were in the non-adenoma group. Imaging characteristics of adenomas (n=128) and non-adenomas (n=64) were compared. Sensitivity and specificity for detection of adenomas were calculated over a range of unenhanced HU values and using absolute washout >60%. Multivariate analysis was performed to identify predictors of adenomas.
Results: Unenhanced HU values <10 were more common in adenomas compared to non-adenomas (47.6% vs. 6.7%, p<0.001), but less than half of the adenomas resected met this criterion. Two non-adenomas (1 lymphangioma and 1 metastasis) measured <3 HU. Non-adenomas were more likely to measure ≥4cm (p=0.001), have irregular borders (p<0.001), have a non-homogeneous appearance (p=0.006), and contain calcifications (p=0.028). These suspicious imaging features were also present in 12-39% of benign adenomas. Multivariate analysis revealed that HU ≤16 (OR 15.9, 95% CI 3.1-81.7, p=0.001) and smooth borders (OR 6.4, 95% CI 2.1-20.0, p=0.001) were both independent predictors of adenomas. The 10 HU cutoff had a sensitivity of 47.6% and a specificity of 93.3% (AUC=0.71, p<0.001). Raising the cutoff to 16 HU improved the sensitivity to 65.9% without detriment to specificity, which remained 93.3% (AUC=0.79, p<0.001). Absolute contrast washout of >60% had a sensitivity and specificity of 53.8% and 100%, respectively (AUC=0.61, p=0.011). In the cohort of patients with washout values available (n=33), if a lesion was <16 HU and/or had >60% absolute washout, the sensitivity and specificity increased to 96% and 100% (AUC=0.98, p<0.001).
Conclusion: The traditional 10 HU threshold has a high specificity for identifying adrenal adenomas, but is limited by a poor sensitivity. Increasing the threshold to 16 HU has the potential to improve sensitivity without sacrificing specificity. A combination criteria of <16 HU and/or >60% absolute washout yielded both a high sensitivity and specificity and can thus be used to accurately identify adrenal adenomas and allow for appropriate selection of patients for non-operative management.