The Bosniak classification for renal cysts: time for another change?

The Bosniak renal cyst classification was introduced in 1986. This classification was based on computed tomography (CT). In the course of years several adjustments were proposed. The current classification contains five categories: I, II, IIF, III and IV. Categorie I and II lesions are assumed benign and do not require further treatment. A IIF lesion is possibly benign and surveillance is advised. Lesions in categorie III and IV are suspect for malignancy and treatment is advised. Only few studies reported interobserver variability which was high. More recent studies incorporate MRI and CEUS in the Bosniak classification system which further impedes interpretation. MRI and CEUS are more sensitive to detect enhancement in a lesion and may upgrade a lesion compared to CT. In Bosniak III cysts the risk for malignancy is approximately 50 %. If malignant, histology usually shows cystic renal cell carcinoma which has been reported to follow a more favourable course compared to solid tumours. In our opinion there are good arguments to imply surveillance for Bosniak III cysts < 4 cm, similar to the approach of IIF lesions. The risk for tumour progression is very small while overtreatment is reduced. It will be important to improve the definition of a category IV lesion.

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