We concur with Dr. Siddiqui's comments that CT scanning is highly sensitive in visualizing intra-adrenal pheochromocytomas [1]. However, CT scanning alone is not sufficient because of its relatively low specificity in distinguishing pheochromocytomas from other adrenal masses such as adrenal incidentalomas [1, 2]. Although MIBG scanning and MRI with T2-weighted images may be more specific for pheochromocytoma [1], diagnosis rests mainly on a high clinical suspicion (particularly in familial syndromes such as von Hippel-Lindau disease) and appropriate biochemical testing.

The disparity in the surgical specimen and CT scan sizes of the left adrenal mass in patient 1 was caused by several factors. A re-evaluation of the original CT scan showed the nodule to be closer to 1.5 cm in greatest diameter. The fixed and sectioned adrenal mass was 1 cm in total diameter, in contrast to 2 cm as noted in the operative report. It was composed of an irregular, nonencapsulated 0.5-cm internal pheochromocytoma component surrounded by a rim of normal adrenal cortical tissue. The slight difference in the total diameter of the mass as measured by CT scanning compared with histologic measurement was probably due to fixation artifact and shrinkage. Although we agree that the "typical" 3- to 5-cm pheochromocytoma is generally encapsulated [3], our experience and that of others show that small, clinically silent lesions may not be encapsulated (Ogorzalek JM. Personal communication).

The CT scan of the abdomen in patient 2 showed a 3.5-cm left adrenal nodule and a normal right adrenal gland. Bilateral adrenalectomy (done after bilateral positive uptake on MIBG scanning) showed a 4.0-cm left adrenal mass and a 2.2-cm right adrenal mass that were both encapsulated and salmon-colored, with adherent rims of residual yellow adrenal cortical tissue. Both masses were histologically confirmed to be pheochromocytomas. Of interest, the CT scan did not show the right adrenal mass. None of our patients have shown any evidence of malignancy.

We agree with Jain and colleagues' suggestion that venous sampling and determination of the adrenal vein noradrenaline-to-adrenaline ratio are valuable [4] for confirming an intra-adrenal pheochromocytoma. However, three of the four patients described by Chew and colleagues [4] had clearly elevated urinary noradrenaline excretion, and venous sampling showed a contralateral second tumor in each. Thus, venous sampling is effective in locating small bilateral pheochromocytomas in patients with elevated 24-hour urinary norepinephrine levels. Our patient 1 had normal urinary and plasma catecholamine values, so we do not know if venous sampling would have shown an intra-adrenal pheochromocytoma. Even in patient 3, venous sampling may not have disclosed a pheochromocytoma at the initial evaluation when the patient was normotensive and had normal biochemical test results. Further studies of adrenal venous sampling are necessary to answer this question.