Adrenal wash-out CT: moderate diagnostic value in distinguishing benign from malignant adrenal masses.

Objective Reliable results of wash-out CT in the diagnostic workup of adrenal incidentalomas are scarce. Thus, we evaluated the diagnostic accuracy of delayed wash-out CT and determined thresholds to accurately differentiate adrenal masses. Design Retrospective, single-center cohort study including 216 patients with 252 adrenal lesions who underwent delayed wash-out CT. Definitive diagnoses based on histopathology (n=92) or comprehensive follow-up. Methods Size, average attenuation values of the adrenal lesions in all CT scan phases, absolute and relative percentage washout (APW/RPW) were determined by an expert radiologist blinded for clinical data. Adrenal lesions with unenhanced attenuation values >10HU built a subgroup (n=142). Diagnostic accuracy were calculated. Results The study group consisted of 171 adenomas, 32 other benign tumors, 11 pheochromocytomas, 9 adrenocortical carcinomas and 29 other malignant tumors. All (potentially) malignant and 46% of benign lesions showed unenhanced attenuation values >10HU. In this most relevant subgroup, the established thresholds of 60% for APW and 40% for RPW misclassified 35.9% and 35.2% of masses, respectively. When we applied optimized cutoffs (APW>83%; RPW>58%) and excluded pheochromocytomas, we missed only 1 malignant tumor by APW and none by RPW. However, only 11% and 15% of benign tumors were correctly identified. Conclusions Washout CT with the established thresholds for APW und RPW is insufficient to reliably diagnose adrenal masses. Using the proposed cutoff of 58% for RPW, malignant tumors will be correctly identified, but the added value is limited, namely 15% of patients with benign tumors can be prevented from additional imaging or even unnecessary surgery.