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With the liberal use of computed tomography (CT) scans and magnetic resonance imaging (MRI), benign renal masses are being detected more frequently. Benign renal tumors include, oncocytoma, angiomyolipoma, leiomyoma, lipoma, hemangioma, and juxtaglomerular tumors.
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Renal oncocytoma has a spectrum of behavior ranging from benign to malignant. Composed of large epithelial cells with finely granular eosinophilic cytoplasm (oncocytes), oncocytomas occur in various organs and organ systems including adrenal, salivary, thyroid, and parathyroid glands as well as the kidney. An estimated 3–5% of renal tumors are oncocytomas (Romis et al, 2004). Men are affected more often than women.
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Renal oncocytomas generally occur and are contained within a well-defined fibrous capsule. Metastasis is extremely rare though invasion of the lymphovascular spaces has been observed. On cut section, the surface of the tumor is usually tan or light brown with a central stellate scar, but necrosis typical of renal adenocarcinoma is absent. The tumors are usually solitary and unilateral, although several bilateral cases and multiple oncocytomas occurring simultaneously (oncocytomatosis) have been reported (Tickoo et al, 1999).
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Oncocytomas can also be associated with benign tumors of hair follicles (fibrofolliculomas), colon polyps/tumors, and pulmonary cysts as part of the Birt–Hogg–Dubé syndrome (Toro et al, 1999). The familial renal oncocytoma syndrome has also been described (Philips et al, 2001). These patients may have a characteristic genetic abnormality involving a gene located on 17p encoding a protein named folliculin (Nickerson et al, 2002) Histologically, well-differentiated oncocytomas are made up of large, uniform cells containing an intensely eosinophilic cytoplasm, which on ultrastructural studies is found to be packed with mitochondria. Mitotic activity is absent, and nuclear pleomorphism is uncommon (Figure 22–1). Consistent chromosomal alterations such as loss of chromosome 1 or Y and translocations in the short arm of chromosome 11 have been described in oncocytomas (Lindgren et al, 2004; Philips et al, 2001). The cellular origin of renal oncocytes has not been fully elucidated, although some early evidence suggested that oncocytes resemble proximal convoluted tubular cells (Merino and Librelsi, 1982). Other findings suggest that their origin may be a precursor stem cell (Cohen et al, 1988) or the intercalated cells of the collecting ducts (Storkel et al, 1989).
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The diagnosis of oncocytoma is predominantly pathologic because there are no reliable distinguishing clinical characteristics. Gross hematuria and flank pain occur in <20% of patients. No characteristic features of the tumors appear on CT, ultrasound (US), intravenous urography (IVU), or MRI. Angiographic features of oncocytomas include the “spoke-wheel” appearance of tumor arterioles, the “lucent rim sign” of the capsule, and a homogeneous capillary nephrogram phase. Unfortunately, these findings are not invariable, and similar findings have been reported in patients with renal cell carcinoma (RCC).
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High-grade oncocytomas may be intermixed with elements of RCC and can be found as coexisting lesions within the same or opposite kidney (Licht et al, 1993). The role of fine-needle aspiration in the preoperative diagnosis of oncocytomas remains ...