Differentiation of Adrenal Hyperplasia From Adenoma by Use of CT Densitometry and Percentage Washout.

OBJECTIVE: The purpose of this study was to quantitatively differentiate adrenal hyperplasia from adenoma with the use of adrenal CT protocols.

MATERIALS AND METHODS: Between January 2004 and November 2012, a total of 156 patients (69 men and 87 women; mean age, 48.9 years) underwent unenhanced CT, early contrast-enhanced CT, and delayed contrast-enhanced CT before undergoing adrenalectomy. Of these patients, 142 had 144 adenomas and 14 had nodular (n = 12) or diffuse (n = 2) hyperplasia. An ROI was placed on 144 adenomas and 27 hyperplastic nodules that were 1 cm or larger or on the four thickest areas of both adrenal glands. The number of each type of lesion and the attenuation value, absolute percentage washout (APW), and relative percentage washout (RPW) associated with hyperplasia and adenoma were compared. The reference standard was pathologic examination. A mixed model and the Fisher exact test were used for statistical analysis.

RESULTS: On unenhanced CT, the mean (± SD) attenuation value for hyperplasia and adenoma was 18.8 ± 10.8 HU and 13.7 ± 15.6 HU, respectively (p = 0.375). When a threshold of 10 HU or lower was used, 22.6% of hyperplastic lesions (7/31) were misdiagnosed as adenomas. The mean APW associated with hyperplasia and adenoma was 73.7% ± 9.3% and 67.3% ± 26.2%, respectively (p = 0.449), whereas the mean RPW for hyperplasia and adenoma was 61.2% ± 9.2% and 59.9% ± 23.0%, respectively (p = 0.625). When an APW threshold of 60% or higher or an RPW threshold of 40% or higher was used, 100% of hyperplastic lesions (31/31) were misdiagnosed as adenoma. Three or more nodules were detected in 33.3% of patients with nodular hyperplasia (4/12) but in none of the patients with adenoma (p < 0.001).

CONCLUSION: Adrenal hyperplasia cannot be quantitatively differentiated from adenoma because there is significant overlap in findings from CT densitometry and assessment of percentage washout. However, the presence of three or more nodules increases the likelihood of nodular hyperplasia.