Renal oncocytoma: misleading diagnosis of benignancy by using angular interface sign at MR imaging

We read with interest the article by Dr Verma and colleagues (1) in the May 2010 issue of Radiology. The authors introduced the angular interface sign as a strong predictor of benignity in exophytic renal masses 2 cm or greater in diameter, with high specificity and diagnostic accuracy. The authors also reported as an important limitation that “the usefulness of this finding may be diminished by the large number of benign lesions.” In the study by Dr Verma and colleagues, the results in renal oncocytoma were not reported. Oncocytoma is a benign neoplasm that comprises approximately 5% of all neoplasms of the renal tubular epithelium in surgical series (2,3). Its incidence is increasing owing to the incidental detection of small renal masses with imaging, and clinically, it may be confused with renal cell carcinoma. Microscopic extension of renal oncocytoma into perinephric adipose tissue may be seen infrequently, and vascular invasion has also been described (4). Its benignancy has been established on the basis of rigorous pathologic review and adequate clinical follow-up (2). In our retrospective review of magnetic resonance (MR) imaging in 29 patients with exophytic renal oncocytomas (2–4.1 cm) that were histologically confirmed with immunostaining, in contrast to the findings reported by Dr Verma and colleagues, we demonstrated that all the lesions had a round interface with the normal renal parenchyma. Therefore, the angular interface sign cannot be considered for diagnosis of all benign renal masses. The angular sign may be a strong predictor of benignancy in an exophytic renal mass (2 cm or greater), with the exception of oncocytoma. The morphologic contour method at unenhanced MR imaging, as proposed by Dr Verma and colleagues, may be used in addition to routine dynamic contrast material–enhanced MR imaging, which remains the most effective method for characterization of a renal lesion (5).