Benign tumors and tumor-like lesions of the adult kidney part II: Benign mesenchymal and mixed neoplasms, and tumor-like lesions

In this review article the benign tumors and tumor-like lesions of the adult kidney are discussed. The incidence of benign renal tumors is low, especially when compared to renal cell carcinomas, as most are detected incidentally or at autopsy. Some of these tumors, as their names imply, are unique to the kidney, e.g., renal adenoma, metanephric adenoma, renal oncocytoma, nephrogenic adenofibroma, mesoblastic nephroma, capsuloma, juxtaglomerular Cell tumor, renomedullary interstitial cell tumor (medullary fibroma), cystic nephroma, cystic partially differentiated nephroblastoma, and cystic hamartoma of the renal pelvis, while others, such as angiomyolipoma, leiomyoma, hemangioma, lipoma, etc., are not unique to the kidney and show similar morphologic features in the other sites they affect. Of the tumor-like lesions, xanthogranulomatous pyelonephritis, malakoplakia, and renal cysts are the most common. The other entities, such as fibroepithelial polyp, are rare, most having been the topic of case reports. In Part I of this paper the benign epithelial tumors of the kidney were previously discussed. This paper (Part II) is devoted to the benign mesenchymal tumors, mixed mesenchymal and epithelial tumors, and the tumor-like lesions.