Differential diagnosis of incidentally detected adrenal masses revealed on routine abdominal CT.

PURPOSE: The aim of this study was to compare CT findings of adrenal incidentalomas with a definitive histological diagnosis in order to establish CT features characteristic for individual types of lesions.

PATIENTS AND METHODS: The retrospective study comprised of patients with adrenal lesions detected on abdominal CT. The patients with these incidental findings subsequently underwent adrenalectomy. The adrenalectomy was followed by a histological assessment of the expansion process. The study consisted of 62 adrenal expansions found in 55 patients (in seven patients bilateral lesions were found). According to the definitive histological diagnosis after adrenalectomy, the lesions were divided into the following six groups: primary adrenocortical carcinoma (n=4), metastasis (n=7), adrenocortical adenoma (n=37), pheochromocytoma (n=9), myelolipoma (n=2), and others (n=3). CT observations were categorized as follows: size, shape, margins, density, side of the expansion, homogeneous or heterogeneous density before and after contrast application, presence of central hypodensity, presence of central calcifications and fat deposits. Statistical analysis was carried out using the chi(2)-test, Kruskal-Wallis test and Mann-Whitney test. To estimate the differences between the subgroups, the t-test was used. For the evaluation of the mutual relations of maximum size, mean size, and volume, regression analysis (coefficient of determination R(2)) was used.

RESULTS: The correlation and regression analysis suggest that there will be no statistically significant error if the maximum size measurements are used instead of the mean size or volume measurements of the lesion. Sensitivity, specificity, accuracy, positive predictive value, negative predictive value for distinguishing adenomas and non-adenomas using a cut-off diameter of 41.5mm were 81.1%, 70.0%, 77.2%, 83.3%, 66.7%, respectively; using the non-contrast density threshold of 23 HU, they were 89.2%, 100%, 93%, 100%, 83.3%, respectively; using the post-contrast density threshold of 47.5 HU, they were 80.6%, 100%, 88.2%, 100%, 76.9%, respectively; using the increase of density threshold of 34.5 HU, they were 74.2%, 70.0%, 72.5%, 79.3%, 63.6%, respectively. A study of receiver operating characteristics (ROC) analyses resulted in the following conclusions: (a) the most accurate parameter for distinguishing adenomas from non-adenomas is the value of non-contrast density, (b) the second most accurate parameter is the post-contrast density, (c) the least suitable parameters are the size of the lesion and increase of density, (d) therefore, in practice, the value of non-contrast density parameter should be used.

CONCLUSION: Standard CT of the abdomen (not specifically aimed at adrenal glands) is a suitable method for distinguishing adrenal lesions which need to be operated on from those which are probably benign but need to be monitored.