Evolution of Bosniak IIF Renal Cysts and Impact of the 2019 Bosniak Classification.

Purpose: The follow-up of Bosniak IIF renal cysts is associated with significant costs, radiation, and anxiety. Recent studies have suggested a risk of malignancy and upgrading lower than previously reported. We aimed to determine their clinical outcomes and to evaluate the impact of the 2019 Bosniak classification on the diagnosis of such lesions.

Materials And Methods: We identified all radiology reports with the diagnosis of a Bosniak IIF cyst at our institution between January 2000 and December 2018. Imaging was reviewed to confirm the diagnosis and determine progression based on the 2005 Bosniak classification. Radiological and clinical characteristics were established, and the 2019 Bosniak criteria were retrospectively applied.

Results: Out of 252 cysts reviewed, 55 (22%) were reclassified as Bosniak II upon revision using the 2005 Bosniak classification. A total of 181 Bosniak IIF cysts were included for final analysis. The median imaging follow-up was 50 months. Four (2.2%) cysts progressed to Bosniak III or IV. Five (2.8%) patients underwent surgical interventions, with only 1 malignant pathology being reported. No malignant progression was observed after 36 months. When applied to our cohort, the 2019 Bosniak classification would have led to a 76% decrease in Bosniak IIF diagnoses, with no increase in Bosniak III or IV diagnoses, and identical classification of the confirmed malignant pathology.

Conclusions: Upgrading and malignancy rates among Bosniak IIF cysts was markedly lower than traditionally reported. No patient had a significant progression beyond 36 months. More than 20% of Bosniak IIF cysts were initially overdiagnosed. The 2019 Bosniak classification may help to reduce the overdiagnosis of Bosniak IIF lesions requiring follow-up.