The CT grey zone in adrenal tumours: Histological and size characteristics of masses with 10–20 HU attenuation

Background On unenhanced CT, attenuation ≤10 Hounsfield units (HU) usually indicates benign adrenal tumours, values >20 HU suggest malignancy, and 10–20 HU defines a grey zone. We evaluated histology and size across HU categories in a laparoscopic adrenalectomy cohort. Methods We retrospectively reviewed 384 adrenalectomies from 2022–2025 (mean age 60.3 years; 66% female). For tumours with unenhanced CT, we recorded HU and size, grouping lesions as <10, 10–20 or >20 HU. Histology was classified as benign, pheochromocytoma/paraganglioma, metastasis or primary malignant; non-benign combined these. Group differences were assessed with chi-square (χ²) tests. Results HU were available in 121/384 tumours and histology in 106/121. Of these, 41 had <10 HU, 30 had 10–20 HU and 35 had >20 HU. In the 10–20 HU group, mean size was 4.2 cm, and histology was benign in 24/30 (80%), pheochromocytoma in 5/30 (17%) and metastasis in 1/30 (3%); no primary malignant lesions. Non-benign lesions increased from 1/41 (2%) in <10 HU to 6/30 (20%) in 10–20 HU and 9/35 (26%) in >20 HU (p=0.01); mean size was similar across groups. Conclusion Most 10–20 HU adrenal tumours were benign despite mean size >4 cm, although non-benign risk was higher than for <10 HU lesions. These findings support cautious use of HU and size thresholds in preoperative counselling for adrenal surgery. Grey-zone attenuation should prompt multidisciplinary review rather than automatic resection. Data collection is ongoing and updated results will be presented at the congress.