Introduction: In surgically managed Bosniak class III cysts, malignancy could be detected in approximately 50%. While Bosniak IV masses are mostly defined to be malignant (approximately 90%). We aimed in this study to delineate if the new Bosniak v2019 classification can predict benign lesions of higher Bosniak classes preoperatively in comparison with the final histopathology to define masses eligible for active surveillance. Methods: We performed a retrospective review between January 2010 and January 2021 for patients who were managed by radical nephrectomy (RN) or nephron-sparing surgery (NSS) for preoperatively diagnosed cystic renal masses (= 25% solid enhancing component). This diagnosis was established after CECT or MRI. Patients’ demographics and radiological data included: age, gender, focal thickening, multilocularity, mass diameter, endophytic or exophytic nature, and final Bosniak v2019 categorization for class III as the following: 1) Enhancing thick wall/septa measuring > 4 mm (III-WS) & 2) Enhancing irregular wall/septa or convex protrusion with obtuse margins measuring <3 mm (III-OP) and for class IV as:1) Enhancing nodule or convex protrusion with obtuse margins measuring >4 mm (IV-OP) and 2) Enhancing nodule or convex protrusion with acute margins of any size (IV-AP). Results: One hundred thirty-seven patients (101 males and 36 females) with higher-stage Bosniak cystic renal masses were included. The mean age for the entire group was 55.5±11.3 years. Bosniak III and IV were identified in 56 and 81 patients, respectively. RN was done in 39 patients. Also, 74.5% of resected masses (102 renal masses) were malignant. In patients who were diagnosed with Bosniak III cyst preoperatively, almost half of the resected masses (46.4%) had benign nature histopathologically. The incidence of malignancy in Bosniak III-WS was 32.1% while, in Bosniak III-OP was 55.3%. In univariate analysis, male gender, exophytic masses, and Bosniak III-OP are predictors for malignant features at the final histopathology (P= 0.008, 0.01, and 0.001, respectively). In multivariate analysis, male gender and Bosniak III-OP were independent risks for malignancy among the resected Bosniak III cysts. On the other hand, in patients with Bosniak IV renal cysts, nine resected masses (11.1%) were benign histopathologically. The incidence of malignancy was 84.7% and 100% in Bosniak IV-OP and AP, respectively. In univariate analysis, male gender, absence of multilocular cysts, and endophytic masses were predictors for malignancy. None of the previous predictors were significant risks for malignancy detection in multivariate analysis. Conclusions: The new version of the Bosniak sub-classification had an important role in defining benign lesions. Bosniak III-OP were independent risks for malignancy detection among the resected Bosniak III cysts. But in general, the new classification lost its sensitivity in malignant masses detection on Bosniak IV cysts. SOURCE OF Funding: none
