FIGURE 1.

The schematic diagram of mixed epithelial and stromal tumor of the kidneys (MESTKs) classified into solid type and cystic type based on the proportion of solid components. A threshold value of 25% is defined in the Bosniak classification (version 2019) of renal cystic masses with solid components, which is the volume proportion of solid components in the mass and the volume is relative to the respective maximal diameter (MD) (V = (4/3)πR ³ and R = MD/2). So, the MD was used to replace the corresponding volume in our study (63% was obtained by the above formula and should be the threshold for the MD ratio of solid part). Note that 60% was used to the threshold of MD proportion of solid components due to the difficulty of measuring septa and nodule less than 5 mm. Therefore, only solid part with diameter more than 10 mm was measured to calculate the ratio (R = (MD_S1 + … + MD_{S n} )/MD_T). It was classified into solid type when R > 60% and cystic type when R ≤ 60%. Some examples with the criterion were illustrated in detail (except for the missing mass with many solid components (n > 5). In Example 1 (case 11), the thickness of multiple regular septa was less than 5 mm and classified into cystic type. In Example 2 (case 5), the MD of the mass (solid line) was 75 mm, and the longest diameter of the solid part (dotted line) was 18.4 mm. So, the ratio is 24.5% (18.4/75) and categorized into cystic type. In Example 3 (case 12), the MD of the mass (solid line) was 67 mm, and the sum of the longest diameter of some solid parts (dotted line) was 38.9 mm (28.7 mm added to 10.1 mm). Thus, the ratio was 57.9% (38.9/67) and classified into cystic type. In Example 4 (case 16), the lesion had huge solid components beyond 60% and belonged to the solid type