The Bosniak classification categorizes renal cystic lesions based on cross-sectional imaging features from clearly benign (Bosniak type I) to malignant lesions (Bosniak type IV). A 67-year-old female patient presented to the emergency department with typical symptoms of acute cholecystitis. During a transabdominal ultrasound examination, an incidental finding was a suspicious cluster of anechoic cystic lesions with internal septa in the left kidney. Following contrast-enhanced computed tomography (CT), the lesion was categorized as a Bosniak type IV cyst. Compared to an earlier CT scan, a Bosniak type I cyst preceded the current Bosniak type IV cyst, suggesting a malignant alteration over the 7-year interval. It was surgically removed, and pathohistological analysis revealed cystic renal cell carcinoma. Although simple renal cysts rarely become malignant, scientific discussion about potential algorithms for additional surveillance is needed.
