Objective: Attenuation values on CT of less than 10 H are considered characteristic of adrenal adenomas. Adrenal pheochromocytomas can infrequently contain fat that could result in low attenuation on CT. The purpose of our study was to determine if pheochromocytomas could be confused with adenomas by virtue of their attenuation values on unenhanced CT.
Materials and methods: CT attenuation and size of nine adrenal nodules producing pheochromocytoma syndrome were measured on unenhanced CT in nine patients. For five patients who received IV contrast material, washout profiles were also calculated.
Results: Two of the nine patients had adrenal lesions with attenuation values of less than 10 H; one had a pheochromocytoma with an attentuation of 9.0 H, and the other had a medullary hyperplasia with an attenuation of 1.8 H. These two nodules showed evidence of microscopic fat at histologic examination. No macroscopic fat was seen on the CT scans. The remaining seven patients had lesions with attenuation values exceeding 10 H (mean value, 25.6 H; range, 1.8-41 H). Mean diameter of the nine tumors (including the hyperplastic nodule) was 3.2 cm (range, 0.8-6.7 cm; SD, +/- 2.3 cm). The two low-attenuation lesions also mimicked adenomas by displaying more than 60% contrast washout on 10-min-delayed contrast-enhanced scans, unlike the other three pheochromocytomas for which we had washout data.
Conclusion: On CT, pheochromocytomas may have attenuation values less than 10 H and also may display more than 60% washout of contrast agents on delayed scanning. Adrenal pheochromocytomas should be included with adenomas in the differential diagnosis both for masses with low attenuation on unenhanced CT and for lesions exhibiting a high percentage of contrast washout.