Purpose: To evaluate the diagnostic yield of multiphasic helical CT in the characterization of single non functioning adrenal nodules (incidentalomas) less than 50 mm in diameter. Emphasis was given to the possible replacement of unenhanced with delayed scans in cancer patients undergoing staging procedures.
Material and methods: Sixty patients with single adrenal nodules (30 of them neoplastic and 30 non-neoplastic) were examined with thin unenhanced scans (5 mm), early scans after administration of a contrast agent (120 mL at 2.5 mL/s with 60 s delay) and late scans (30 min delay).
Results: On both unenhanced and late scans a threshold could be selected on the Hounsfield unit scale which guaranteed absolute specificity in the characterization of adenomas (100% specificity) with very high sensitivity (93% at both scans): this threshold was 19 HU on unenhanced and of 41 HU on late scans. In contrast, at early delayed scanning the threshold which guaranteed 100% specificity was associated with negligible sensitivity (30%). The evaluation of lesion size had no diagnostic value, since the mean diameter of both benign and malignant nodules was of 25 mm.
Conclusions: Late scans have diagnostic yield comparable to unenhanced scans: at the selected delay (30 min), benign lesions nearly always have lower attenuation values than malignant nodules and can be diagnosed with confidence when they exhibit mean attenuation values lower than 41.