Metanephric adenoma: case report and review of the literature

Metanephric adenomas are benign tumors frequently found post-mortem (from 7% to 22% of autopsies) which originate from distal tubules; they are generally small in dimensions (smaller than 1 cm) and located in the renal cortex. Etiology is unknown, even though they could be associated with smoke, tubular nephrosclerosis, dialysis. An endocrinal relationship was proposed, because of its more frequent incidence in female (2:1). Metanephric adenoma is an uncommon kind of renal adenoma with no malignant potentiality: from the clinical and diagnostic viewpoint its own greater importance depends on the probability of diagnostic misunderstanding with Wilms' tumor; furthermore its echographic, tomographic and arteriographic characteristics are often similar to small renal adenocarcinoma ones (100). Polycytemia is frequently associated to metanephric adenoma as paraneoplastic syndrome and, more rarely, abdominal mass, abdominal pain, hematuria and hypertension (140). The most important study on metanephric adenoma is the one realized by E.K. Mostofi, including 50 cases from the Department of Genitourinary Pathology, Armed Forces Institute of Pathology, Washington, D.C., published in 1995; in this study half of findings was incidental; mean dimension of tumor about 5,5 cm and in 50% it could be seen a distinct capsule sourrounding the tumor (in the remaining cases the capsule was discontinuos or absent) (210). An important radiological characteristic is that metanephric adenomas are more frequent calcificated than other histotypes and, from the pathological viewpoint, the most important differential element seems to be the smaller dimensions of its cells with hyperchromatic nuclei in the absence or lack in mitotic activity and in the absence of chromosome aberrations. In the case of difficult histological diagnosis, cytogenetic and immunohistochemical analysis can be useful. In conclusion, because it is impossible to distinguish in the preoperatory between the metaneprhic adenomas and the other tumoral types on the bases of the symptoms, dimensions or radiographic appearance, it is mandatory to treat it as malignant eteroformations in a therapeutical strategy, when it is possible.