Bosniak Classification version 2019: validation and comparison to original classification in pathologically confirmed cystic masses

Objective To evaluate Bosniak Classification v2019 definitions in pathologically confirmed cystic renal masses. Materials and methods Seventy-three cystic (≤ 25% solid) masses with histological confirmation (57 malignant, 16 benign) imaged by CT ( N = 28) or CT+MRI ( N = 56) between 2009 and 2019 were independently evaluated by three blinded radiologists using Bosniak v2019 and original classifications. Discrepancies were resolved by consensus with a fourth blinded radiologist. Overall class and v2019 features were compared to pathology. Results Inter-observer agreement was slightly improved comparing v2019 to Original Bosniak Classification (kappa = 0.26–0.47 versus 0.24–0.34 respectively). v2019 proportion of IIF and III masses (20.5% [15/73, 95% confidence interval (CI) 12.0–31.6%], 38.6% [28/73, 95% CI 27.2–50.5%]) differed from the original classification (6.8% [5/73, 95% CI 2.3–15.3%], 61.6% [45/73, 95% CI 49.5–72.8%]) with overlapping proportion of malignancy in each class. Mean septa number (7 ± 4 [range 1–10]) was not associated with malignancy ( p = 0.89). Mean wall and septa thicknesses were 3 ± 3 (1–14) and 3 ± 2 (1–10) mm and higher in malignancies ( p = 0.03 and 0.20 respectively). Areas under the receiver-operator-characteristic curve for wall and septa thickness were 0.66 (95% CI 0.54–0.79) and 0.61 (95% CI 0.45–0.78) with an optimal cut point of ≥ 3 mm (sensitivity 33.3%, specificity 86.7% and sensitivity 53%, specificity 73% respectively). Proportion of malignancy occurring in masses with the v2019 features “irregularity” (76.9% [10/13], 95% CI 46.2–94.9%) and “nodule” (89.7% [26/29], 95% CI 72.7–97.8%) overlapped. Angle of “nodule” ( p = 0.27) was not associated with malignancy. Conclusion Bosniak v2019 definitions for wall/septa thickness and protrusions are associated with malignancy. Overall, Bosniak v2019 categorizes a higher proportion of malignant masses in Class IIF with slight improvement in inter-observer agreement. Key Points • Considering Bosniak v2019 Class IIF cystic masses with many (≥ 4) smooth and thin septa, there was no association between the number of septa and malignancy (p = 0.89) in this study. • Increased cyst wall and septa thickness are associated with malignancy and a lower threshold of ≥ 3 mm maximized overall diagnostic accuracy compared to ≥ 4 mm threshold proposed for Bosniak v2019 Class 3. • An overlapping proportion of malignant masses is noted in Bosniak v2019 Class 3 masses with “irregularity” (76.9% [10/13], 95% CI 46.2–94.9%) compared to Bosniak v2019 Class 4 masses with “nodule” (89.7% [26/29], 95% CI 72.7–97.8%).