Oncocytoma Definition: Oncocytoma is a benign renal tumor that arises from the intercalated cells of the kidney’s collecting ducts. It accounts for about 3-7% of all renal tumors and is often discovered incidentally during imaging studies for unrelated conditions. Although benign, oncocytomas can grow quite large and may be mistaken for malignant renal cell carcinoma (RCC) on imaging studies. Epidemiology: Oncocytomas are most commonly diagnosed in adults, typically between the ages of 50 and 70 years, and they have a slight male predominance. They are usually solitary lesions, though in some cases, particularly in patients with Birt-Hogg-Dubé syndrome, they may be multiple. Oncocytomas are not associated with any specific environmental or lifestyle risk factors. Pathogenesis: The exact cause of oncocytoma is unknown, but it is believed to arise from the oncocytes, which are epithelial cells characterized by an abundance of mitochondria. This mitochondrial proliferation is thought to be a key feature in the pathogenesis of oncocytomas. Genetic studies have identified various chromosomal abnormalities in oncocytomas, including loss of chromosomes 1 and Y, but these findings are not consistent across all cases. Histopathology: Histologically, oncocytomas are composed of large, eosinophilic cells with abundant granular cytoplasm due to the high concentration of mitochondria. The tumor is usually well-circumscribed and may show a characteristic central stellate scar. Oncocytomas are typically unencapsulated and can be distinguished from RCC by their lack of significant nuclear atypia, mitotic activity, and necrosis. Immunohistochemical staining can also aid in diagnosis, with oncocytomas often positive for markers like CK7 (cytokeratin 7) and negative for CD10 and AMACR, which are more commonly seen in RCC. Clinical Presentation: Most patients with oncocytomas are asymptomatic, and the tumor is often found incidentally during imaging for other reasons. When symptoms do occur, they may include hematuria, flank pain, or a palpable abdominal mass, particularly if the tumor is large. Due to its benign nature, oncocytomas typically do not metastasize or invade surrounding structures. Diagnosis: Oncocytomas are often difficult to distinguish from RCC based on imaging alone. Features such as a central scar seen on CT or MRI may suggest oncocytoma, but this is not definitive. The definitive diagnosis usually requires histological examination, either through biopsy or after surgical resection. Fineneedle aspiration cytology may also be used, but it can be challenging to differentiate oncocytoma from other renal neoplasms with similar cytological features. Treatment: The treatment of choice for oncocytoma is typically surgical excision, particularly if the tumor is large or symptomatic. In cases where the diagnosis is uncertain, partial nephrectomy or nephron-sparing surgery is preferred to preserve renal function. For small, asymptomatic lesions, active surveillance may be an option, especially in older patients or those with significant comorbidities. Prognosis: The prognosis for patients with oncocytoma is excellent, given its benign nature and lack of metastatic potential. Recurrence after surgical resection is rare, and long-term follow-up is generally not necessary unless the patient has a predisposition to developing multiple renal tumors, such as in the context of a hereditary syndrome.