The Malignant Imposter: Cystic Renal Oncocytoma Challenging Bosniak Criteria.

Renal cystic lesions are frequently encountered in clinical practice. While simple cysts are benign, complex cysts with solid components (e.g., Bosniak III/IV) often raise concerns regarding malignancy. Rarely, benign renal tumors such as oncocytoma may present with cystic changes, posing a significant diagnostic challenge. A 70-year-old male was found to have a right renal cyst on abdominal imaging conducted for right upper quadrant fullness. Contrast-enhanced CT tomography of the abdomen revealed a 6.8 x 6.6 x 6.4 cm exophytic hypodense cystic lesion with an internal soft tissue component measuring 13 mm in thickness, which exhibited significant enhancement after contrast administration. Due to the suspicion of cancer, a right partial nephrectomy was performed. The patient had a smooth postoperative recovery. Histopathological analysis confirmed the lesion as a renal oncocytoma with cystic alterations. Immunohistochemistry tests were negative for CK7, confirming the diagnosis of renal oncocytoma. This case highlights the importance of histopathological and immunohistochemical assessments and emphasizes the need to prevent overtreatment of patients.