CT of adrenal tumors: frequency and clinical significance of low-attenuation lesions.

The CT values of adrenal tumors were evaluated in 36 patients to determine the frequency and significance of negative CT values. Twenty-seven patients had adrenocortical adenomas (13 had primary aldosteronism, 12 had Cushing syndrome, and two had nonhyperfunctioning adenoma), one had adrenocortical carcinoma, and eight had pheochromocytomas. The CT values in primary aldosteronism (mean, 1.8 +/- 9.9 H) were lower than those in Cushing syndrome (27.6 +/- 12.0 H) and pheochromocytoma (35.9 +/- 9.8 H). Six adrenocortical adenomas had homogeneous, low CT attenuation, with CT values ranging from -20 to -0.4 H. Four of these had primary aldosteronism, one had Cushing syndrome, and one had nonhyperfunctioning adenoma. Our results suggest that adrenal tumors with homogeneous, slightly negative CT attenuation are likely to be adrenocortical adenomas, mainly primary aldosteronism and nonhyperfunctioning adenomas. This finding may be especially useful in diagnosing nonhyperfunctioning adenoma.