Introduction: In 2019, a major revision of the Bosniak classification was proposed to address several shortcomings. The purpose of this systematic review is to determine the proportion of malignant renal masses within newly described Bosniak v2019 class 2 cystic masses for CT and MRI.

Methods: Multiple databases were searched. Studies were eligible for inclusion if they evaluated malignancy rate of cystic renal masses using any of the Bosniak v2019 class 2 subtypes. Risk of bias and applicability was assessed. Proportion of malignant renal masses for each included study was calculated.

Results: 12 studies met inclusion criteria. 2146 renal masses were evaluated within five Bosniak class 2 subtypes reporting rates of malignancy as follows: 1)‘homogeneous hyperattenuating (≥ 70 HU) masses at noncontrast CT’; 0% in all studies 2)‘homogeneous masses -9 to 20 HU at noncontrast CT’; 0% in all studies 3)‘homogeneous masses 21 to 30 HU at portal venous phase CT’; 0% in all studies, 4)‘homogeneous masses markedly hyperintense at T1-weighted imaging (approximately 2.5 normal parenchymal signal intensity) at noncontrast MRI’; range of 0% to 15% and 5) ‘homogeneous masses markedly hyperintense at T2-weighted imaging (similar to CSF) at noncontrast MRI’; 0% in one study. No studies evaluated the other Bosniak class 2 subtypes: ‘cystic masses with thin (≤ 2 mm) and few (1–3) septa; septa and wall may enhance; may have calcification of any type’ for either CT or MRI and ‘homogeneous low-attenuation masses that are too small to characterize’ for CT. Forest plot of results summarized in Figure 1.

Conclusions: Bosniak v2019 class 2 cystic renal masses are highly likely to be benign cysts with similar or lower proportion of malignancy compared to the original Bosniak classification. Relatively few studies evaluated renal masses within each class 2 subtype, and some were at risk of bias, necessitating future validation of each subtype described in class 2 of the Bosniak v2019 update.