Incidental Adrenal Nodules and Growth Rates: A Single Centre Retrospective Cohort Study

Incidental adrenal nodules are increasingly being detected due to the widespread use of computed tomography (CT) and advances in imaging technology. Current guidelines often recommend follow-up imaging around one year, but, given the poor prognosis of malignant adrenal lesions, the utility of this approach is unclear. Therefore, we investigated whether growth rates of incidental adrenal nodules are consistent with this guidance.

We conducted a retrospective cohort study at Te Toka Tumai Auckland, New Zealand. We identified all CT adrenal examinations performed between Jan 2010 and Dec 2015 in which the patient had an incidental adrenal nodule on prior CT and who had at least one follow-up scan. Annual nodule growth rates were calculated and compared between malignant and benign lesions.

66 patients with 75 adrenal nodules were identified. The mean (SD) age was 63.5 (10.7) years; 44% were female; average nodule size was 21.2 (10.5) mm; and average follow-up time was 5.8 (2.8) years. The solitary malignant nodule rapidly increased in size (13 mm/39 days). Of the 74 benign nodules, 32 (43%) increased in size (mean annual growth 0.9 mm/year), 24 (32%) decreased in size (mean -0.7 mm/year), and 18 (24%) were unchanged at final follow-up. 95% of benign nodules had an annual growth rate between -2 and +2 mm/year.

Malignant lesions grow at an order of magnitude higher than benign lesions, which mostly remain stable or grow very slowly. A one-year follow-up scan may be too late to be of clinical use in malignant lesions. Follow-up should be based on the risk of malignancy at baseline and/or the growth rate on any subsequent adrenal imaging.