Histological spectrum of resected adrenal tumors in the 11–20 HU grey zone on unenhanced CT: a retrospective cohort study

Adrenal tumors with unenhanced attenuation of 11-20 Hounsfield units (HU) are indeterminate on noncontrast computed tomography (CT). This interval is clinically relevant because it usually triggers additional biochemical, radiological, and multidisciplinary assessment rather than an immediate decision to operate or observe. We evaluated the pathology-confirmed histological spectrum of resected adrenal lesions across guideline-consistent attenuation categories, with particular focus on the 11-20 HU grey zone. We retrospectively reviewed 383 adults who underwent minimally invasive adrenalectomy between 2022 and 2025. Unenhanced CT attenuation values were recorded in 202 cases; two necrotised-tissue cases were excluded from the HU-based histology analysis. The final analytic cohort comprised 200 tumors classified as ≤ 10 HU, 11-20 HU, or > 20 HU. The primary outcome was final histopathology grouped as benign lesion, pheochromocytoma/paraganglioma (PPGL), metastasis, or primary malignant lesion. Clinical background was reviewed for the five non-benign 11-20 HU lesions. Exploratory multivariable logistic regression evaluated non-benign versus benign histology. The final cohort included 82 tumors with ≤ 10 HU, 44 with 11-20 HU, and 74 with > 20 HU. In the 11-20 HU group, histology was benign in 39/44 lesions (88.6%), PPGL in 3/44 (6.8%), metastasis in 1/44 (2.3%), and adrenocortical carcinoma in 1/44 (2.3%). Non-benign pathology increased from 4.9% at ≤ 10 HU to 11.4% at 11-20 HU and 33.8% at > 20 HU. In multivariable analysis, > 20 HU was associated with non-benign histology compared with ≤ 10 HU (odds ratio [OR] 10.0, 95% confidence interval [CI] 3.18-31.30), whereas 11-20 HU was not independently associated with a statistically significant increase (OR 2.81, 95% CI 0.69-11.50). Larger tumor size was also associated with non-benign histology (OR 1.30 per cm, 95% CI 1.09-1.56). Most resected adrenal tumors in the 11-20 HU grey zone were benign, but this interval still contained clinically important non-benign pathology. These findings should be interpreted as surgical-cohort data, not as prevalence estimates for true adrenal incidentalomas. Intermediate attenuation should be used as a prompt for integrated biochemical, imaging, oncological, and multidisciplinary assessment rather than as an isolated indication for adrenalectomy.