Introduction
Renal oncocytoma is a benign epithelial neoplasm arising from the intercalated cells of the distal renal tubules. Despite its benign nature, it may represent a diagnostic challenge because of its close morphological resemblance to certain malignant renal tumors, particularly the eosinophilic variant of chromophobe renal cell carcinoma. Accurate diagnosis therefore requires careful correlation between clinical presentation, imaging findings, histopathological examination, and immunohistochemical analysis.

Methods
We report the case of a 43-year-old woman with no significant past medical history who presented with intermittent right-sided lumbar pain evolving for approximately one year. Physical examination revealed a hemodynamically stable and afebrile patient with a soft abdomen and mild diffuse tenderness.

Results
Abdominal computed tomography initially revealed a small indeterminate lesion in the mid-portion of the right kidney. Subsequent magnetic resonance imaging demonstrated a well-circumscribed cortico-medullary mass located at the upper pole of the right kidney, confined to the renal parenchyma without vascular invasion, extrarenal extension, or regional lymphadenopathy. A partial right nephrectomy was therefore performed.

Macroscopic examination showed a surgical specimen measuring 2.4 × 2 cm containing a well-defined, firm, brownish nodule measuring 2 × 1.4 × 0.9 cm, located 2 mm from the nearest surgical margin. The renal capsule was intact with focal hemorrhagic areas.

Microscopically, the tumor consisted of large eosinophilic cells with finely granular cytoplasm arranged in nests and trabeculae. No significant nuclear atypia, mitotic activity, or necrosis was observed. Immunohistochemistry revealed diffuse CD117 and cyclin D1 expression, with negative AMACR and absent or focal CK7 staining, supporting the diagnosis of renal oncocytoma.

Conclusion
Renal oncocytoma is a benign tumor with an excellent prognosis; however, its close resemblance to malignant renal neoplasms may complicate diagnosis. Careful histopathological evaluation supported by immunohistochemistry is essential for accurate classification and appropriate clinical management.