Introduction: The followup of Bosniak BIIF renal cysts is associated with significant costs, radiation, and anxiety. Recent studies suggest a risk of malignancy and upgrading lower than previously reported. New radiological definitions of the Bosniak categories were introduced in 2019. We aimed to determine the radiological and clinical evolution of BIIF cysts diagnosed at our institution and to establish the impact of the 2019 Bosniak classification on the diagnosis of such lesions.

Methods: We identified all radiology reports with the diagnosis of a BIIF cyst at our institution between January 2000 and December 2018. Diagnostic and followup imaging were reviewed by trained radiologists to confirm the diagnosis and determine progression. Radiological and clinical characteristics were established, and the 2019 Bosniak criteria were retrospectively applied.

Results: Of 252 cysts initially reviewed, 55 (22%) were re-classified as BII upon revision. A total of 181 BIIF cysts were included for final analysis. The median imaging followup was 50 months. Only four cysts (2%) progressed to BIII or BIV. Five patients (3%) underwent surgical interventions, with only one malignant pathology being reported. No patient had a radiological progression without a confirmed benign pathology beyond 36 months. When applied to our cohort, the 2019 Bosniak classification would have led to a 76% decrease in BIIF diagnoses, with no increase in BIII or BIIV diagnoses, and identical classification of the confirmed malignant pathology.

Conclusions: The rate of upgrading and malignancy among BIIF renal cysts was markedly lower than traditionally reported. No patient had a significant progression beyond 36 months. More than 20% of BIIF cysts were initially overdiagnosed by radiologists. The 2019 Bosniak classification may help to reduce the overdiagnosis of BIIF lesions requiring followup, avoiding important costs/harm to patients.