Defining new radiological patterns to improve classification of Bosniak III and IV cystic renal masses.

We aimed to compare Bosniak III and IV cystic renal masses (CRM) using Bosniak classification version 2005 (BC-v2005) versus BC-v2019 and analyze radiological findings and patterns of benignity and malignancy. We retrospectively reviewed all Bosniak III-IV CRMs using BC-v2005 at our center with four-phase CT confirmed pathology, between January 2014 and June 2019. Two radiologists independently re-evaluated each lesion using both BCs, including findings of benignity and malignancy. Bosniak III-IV CRMs were classified in four radiological patterns: inflammatory (CRM-IP), cystic nephroma (CRM-CNP), papillary (CRM-PP), and clear cell (CRM-CCP). Characteristics of patterns were compared. Out of 97 patients, 57 Bosniak III-IV CRMs met the inclusion criteria. Twenty-four (42.10%) lesions were reclassified as solid tumours using BC-v2019. Any lesion was downgraded to IIF or lower. Nine (15.78%) lesions were upgraded to Bosniak III-IV CRMs by BC-v2019, despite benign lesions. The presence of acute-angle nodules was the only statistically significant sign of malignancy (p < 0.01). All benign lesions, not considered solid, were included in the two patterns of benignity, characterized by thickened septa and/or wall and without acute-angle nodules. All malignant CRMs presented two patterns of malignancy, proposed as acute-angle nodules. All clear cell carcinomas presented with hyperenhancing acute-angle nodules in the arterial phase. Pattern classification was better for BC-v2005 and BC-v2029, differentiating benign and malignant CRM. The proposed four radiological patterns improved performance than BC-v2019 in classifying benign lesions (CRM-IP and CRM-CNP) and malignant ones (CRM-PP and CRM-CCP), with wall and septa thickening and acute-angle nodules as relevant biomarkers.