Prevalence of Malignancy Among Incidental Indeterminate Adrenal Nodules on Contrast-Enhanced CT in Patients Without Known Cancer: A Multiinstitutional Study.

BACKGROUND. Adrenal incidentalomas are common findings at contrast-enhanced CT, yet their management remains controversial. OBJECTIVE. The purpose of this study was to determine the prevalence of malignancy among incidental indeterminate adrenal nodules detected on contrast-enhanced CT in patients without known cancer. METHODS. We performed a 12-institution retrospective cohort study of adult patients without known cancer who underwent contrast-enhanced CT of the abdomen from January 1, 2010, to April 2, 2016. Consecutive reports from 10,646 cases were reviewed. CT images were reviewed for 2603 cases with reports describing an adrenal nodule measuring 1 cm or larger. Malignancy or benignancy was determined using the following reference standards: pathology, diagnostic imaging (attenuation on unenhanced CT < 10 HU or signal-intensity loss at chemical-shift MRI), imaging stability for at least 1 year, or clinical follow-up of at least 5 years. Descriptive statistics were performed using the binomial exact method and chi-square test. RESULTS. The final cohort included 1320 patients (813 women, 507 men; mean age, 63.1 ± 15.2 [SD] years) with 1506 adrenal nodules. Mean nodule size was 2.1 ± 0.7 (SD) cm (range, 1.0-10.7 cm; IQR, 1.6-2.5 cm). The prevalence of malignancy among all nodules, nodules measuring 1-2 cm, nodules measuring 2-4 cm, and nodules measuring more than 4 cm was 0.1% (1/1506; 95% CI, 0.0-0.4%), 0.0% (0/773; 95% CI, 0.0-0.4%), 0.1% (1/694; 95% CI, 0.0-0.8%), and 0.0% (0/39; 95% CI, 0.0-7.4%), respectively. The one malignant nodule was an adrenocortical carcinoma diagnosed 9 years after initial detection. A total of 940 nodules underwent unenhanced CT after the index CT examination; all were benign. Of these, 654 were less than 10 HU, 157 were 10-20 HU, and 129 were more than 20 HU. CONCLUSION. The prevalence of malignancy among incidental indeterminate adrenal nodules on contrast-enhanced CT in patients without known cancer was exceedingly low. CLINICAL IMPACT. Follow-up imaging is not warranted for small (1-2 cm) incidental indeterminate adrenal nodules detected on contrast-enhanced CT in patients without known cancer.