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Bosniak Classification of Renal Cystic Lesions

Grade Description
Grade I Simple cyst, benign. Features: CT ① Round, no wall; ② Uniform water-like density (CT value 0-20HU); ③ Clear boundary, smooth and sharp edges; ④ No enhancement on enhanced scan;
Grade II Mildly complex cyst (including septated cysts, benign small calcified cysts, infected cysts, high-density cysts). CT features: ① Thin and uniform cyst wall, few (less than 2) and small (less than 1mm) and regular septa. ② Small calcifications in the cyst wall or septa. ③ Slight enhancement of the cyst wall or septa;
Grade IIF (F: follow up) Moderately complex cyst. CT diagnostic criteria: ① Uniform thickening of the cyst wall and septa. ② Increased calcifications. ③ Mild enhancement of the cyst wall and septa. ④ High-density cysts with a diameter of ≥3cm and completely located within the renal parenchyma. The lesion has some features of both Grade II and III but does not fully meet the diagnostic criteria for either.
Grade III More complex cyst, indeterminate, including benign and malignant (e.g., multilocular cystic nephroma, complex septated cyst, chronic infected cyst, calcified cyst; cystic renal carcinoma). CT features: ① Thick (greater than 1mm) and irregular cyst wall or septa. ② Increased septa (≥3). ③ Calcifications in the cyst wall or septa, more calcifications, small solid components in the cyst wall. ④ Significant enhancement of the septa or cyst wall. ⑤ Some are benign lesions;
Grade IV Definite malignant cystic mass, mainly cystic renal carcinoma. CT features: ① Features of Grade III cysts. ② Independent soft tissue components adjacent to the cyst wall or septa.

Explanation

Grades I and II are benign and do not require surgery or follow-up. Grades III and IV require surgical removal. Grade IIF requires follow-up to determine its biological behavior.

References

Research Paper 1. Bosniak MA. The current radiological approach to renal cysts. Radiology 1986;158:1–10.
Research Paper 2. Bosniak MA. The Bosniak renal cyst classification: 25 years later. Radiology. 2012. 262 (3): 781-5.
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