Small (< 4 cm) Bosniak renal cysts: association of initial fluid percentage and enhancing solid volume thresholds with future lesion behavior

In Pursuit of KI-RADS: Toward a Single, Evidence-based Imaging Classification of Renal Masses

Despite the successful application of Imaging Reporting and Data Systems to improve the radiologic description and management of disease in many organs, one does not yet exist for the kidney. Instead, the radiologic approach to the kidney has focused on the Bosniak classification system, which is based on imaging characteristics for cystic renal masses, and detecting macroscopic fat within solid renal masses. Radiologically, cystic and solid renal masses are categorized and evaluated separately because of historical precedent, differences in appearance at imaging, and differences in biologic behavior. However, the World Health Organization classification of renal neoplasms does not support such separation. Further, the primary goal has been cancer diagnosis. Differentiating benign from malignant masses is important, but data show that many renal cancers, particularly when small, will not cause harm. Therefore, a critical goal of any unifying, single, imaging-based classification of kidney masses (ie, a Kidney Imaging Reporting and Data System) should be predicting the biologic behavior or aggressiveness of suspected kidney cancer. This system could inform the need for treatment or active surveillance and reduce prevalent overdiagnosis and overtreatment. This review describes the rationale for and challenges in creating such a system and the research needed for it to be developed.

Clinical value of contrast-enhanced ultrasound combined with quantitative analysis in Bosniak ≥ II cystic renal masses

OBJECTIVE

The 2019 Bosniak classification has improved the precise and detailed quantitative evaluation criteria, making the diagnosis of cystic renal masses (CRMs) more accurate and objective. This study addresses the clinical value of quantitative analysis and aims to investigate the feasibility of combining contrast-enhanced ultrasound (CEUS) with quantitative analysis for diagnosing Bosniak ≥ II CRMs.

METHODS

We retrospectively obtained 58 CRMs with confirmed pathology, which underwent CEUS and Contrast-enhanced computer tomography (CECT) evaluations according to Bosniak classification between January 2013 and August 2024. These lesions were divided into benign and malignant groups, followed by a quantitative analysis of the morphological details detected by CEUS. All morphological parameters were compared, and the diagnostic efficiencies were evaluated using receiver operating characteristics (ROC) curves, logistic regression (LR) analysis, and diagnostic curve analysis (DCA). Additionally, a cohort of 72 lesions was monitored for a period of ≥ 3 years, and changes in Bosniak classification were analyzed by categorizing them into stable, upgraded, and downgraded categories.

RESULTS

The analysis revealed no statistically significant difference between CEUS and CECT in our cohort's malignancy predictive rates across different Bosniak grades (p = 0.640). All morphological quantitative parameters showed statistically significant differences between the two groups (p < 0.001). ROC curve analysis revealed that the sum of enhanced wall thickness and enhanced septum thickness among quantitative parameters had the highest AUC value (AUC: 0.9226). Both LR models demonstrated superior clinical diagnostic performance with similar level of accuracy between qualitative and quantitative analysis, as evidenced by ROC (AUC: 0.9470, 0.9619, respectively) and DCA analyses. None of the lesions in the follow-up cohort were upgraded, suggesting that CRMs are relatively stable tumors with a low malignant potential.

CONCLUSION

This retrospective study demonstrated that CEUS combined with Bosniak classification and quantitative analysis could enhance diagnostic confidence in differentiating Bosniak ≥ II CRMs and could serve as a viable alternative to CECT in specific cases.

Bosniak Classification of Cystic Renal Masses: Looking Back, Looking Forward

RATIONALE AND OBJECTIVES

According to the 2019 update of the Bosniak classification, the main imaging features that need to be evaluated to achieve a correct characterization of renal cystic masses include the thickness of walls and septa, the number of septa, the appearance of walls and septa, the attenuation/intensity on non-contrast CT/MRI and the presence of unequivocally perceived or measurable enhancement of walls and septa. Despite the improvement deriving from a quantitative evaluation of imaging features, certain limitations seem to persist and some possible scenarios that can be encountered in clinical practice are still missing.

MATERIALS AND METHODS

A deep analysis of the 2019 update of the Bosniak classification was performed.

RESULTS

The most notable potential flaws concern: (1) the quantitative measurement of the walls and septa; (2) the fact that walls and septa > 2 mm are always referred to as "enhancing", not considering the alternative scenario; (3) the description of some class II masses partially overlaps with each other and with the definition of class I masses and (4) the morphological variations of cystic masses over time is not considered.

CONCLUSION

The present paper analyzes in detail the limitations of the 2019 Bosniak classification to improve this important tool and facilitate its use in daily radiological practice.

The efficacy of the new Bosniak classification v.2019 in benign lesions prediction within the higher Bosniak cysts classes

INTRODUCTION

Identification of benign lesions among higher classes of renal Bosniak cysts who are vulnerable to active surveillance instead of surgical approach is still questionable. We aimed in this study to delineate the efficacy of the new Bosniak v2019 classification in benign lesions identification among those cases with higher Bosniak classes in comparison with the final histopathology.

MATERIALS

In a retrospective review between 2010 and 2021 for patients diagnosed as higher classes Bosniak renal masses was done. Patients' demographics and radiological data i.e.,: age, gender, and final Bosniak v2019 categorization for class III: (1) Enhancing thick wall/septa >4 mm (III-WS) and (2) Enhancing irregular wall/septa or convex protrusion with obtuse margins <3 mm (III-OP) and for class IV as: (1) Enhancing nodule or convex protrusion with obtuse margins >4 mm (IV-OP) and (2) Enhancing nodule or convex protrusion with acute margins of any size (IV-AP).

RESULTS

A total of 137 patients were included. Bosniak III was identified in 56 patients. Malignancy was detected in 74.5% of resected masses. Among resected Bosniak III cyst, 46.4% were benign histopathologically. Male gender and Bosniak III-OP were independent risks for malignancy among the resected Bosniak III cysts. Conversely, in resected Bosniak IV renal cysts, only 9 of resected masses were benign. In univariate analysis, male gender, absence of multilocular cyst and endophytic masses were predictors for malignancy in resected Bosniak IV cyst. None of the previous predictors was significant in multivariate analysis.

CONCLUSION

The Bosniak subclassification v.2019 can define benign lesions. Bosniak III-OP was an independent risk for malignancy detection among the resected Bosniak III cysts.

Growth Kinetics and Progression Rate of Bosniak Classification Version 2019 Class III and IV Cystic Renal Masses on Imaging Surveillance

BACKGROUND. Active surveillance is increasingly used as first-line management for localized renal masses. Triggers for intervention primarily reflect growth kinetics, which have been poorly investigated for cystic masses defined by the Bosniak classification version 2019 (v2019). OBJECTIVE. The purpose of this study was to determine growth kinetics and incidence rates of progression of class III and IV cystic renal masses, as defined by the Bosniak classification v2019. METHODS. This retrospective study included 105 patients (68 men, 37 women; median age, 67 years) with 112 Bosniak v2019 class III or IV cystic renal masses on baseline renal mass protocol CT or MRI examinations performed from January 2005 to September 2021. Mass dimensions were measured. Progression was defined as any of the following: linear growth rate (LGR) of 5 mm/y or greater (representing the clinical guideline threshold for intervention), volume doubling time less than 1 year, T category increase, or N1 or M1 disease. Class III and IV masses were compared. Time to progression was estimated using Kaplan-Meier curve analysis. RESULTS. At baseline, 58 masses were class III and 54 were class IV. Median follow-up was 403 days. Median LGR for class III masses was 0.0 mm/y (interquartile range [IQR], -1.3 to 1.8 mm/y) and for class IV masses was 2.3 mm/y (IQR, 0.0-5.7 mm/y) (p < .001). LGR was at least 5 mm/y in four (7%) class III masses and 15 (28%) class IV masses (p = .005). Two patients, both with class IV masses, developed distant metastases. Incidence rate of progression for class III masses was 11.0 (95% CI, 4.5-22.8) and for class IV masses 73.6 (95% CI, 47.8-108.7) per 100,000 person-days of follow-up. Median time to progression was undefined for class III masses given the small number of progression events and 710 days for class IV masses. Hazard ratio of progression for class IV relative to class III masses was 5.1 (95% CI, 2.5-10.8; p < .001). CONCLUSION. During active surveillance of cystic masses evaluated using the Bosniak classification v2019, class IV masses grew faster and were more likely to progress than class III masses. CLINICAL IMPACT. In comparison with current active surveillance guidelines that treat class III and IV masses similarly, future iterations may incorporate relatively more intensive surveillance for class IV masses.

Active Surveillance of Renal Masses: The Role of Radiology

Active surveillance of renal masses, which includes serial imaging with the possibility of delayed treatment, has emerged as a viable alternative to immediate therapeutic intervention in selected patients. Active surveillance is supported by evidence that many benign masses are resected unnecessarily, and treatment of small cancers has not substantially reduced cancer-specific mortality. These data are a call to radiologists to improve the diagnosis of benign renal masses and differentiate cancers that are biologically aggressive (prompting treatment) from those that are indolent (allowing treatment deferral). Current evidence suggests that active surveillance results in comparable cancer-specific survival with a low risk of developing metastasis. Radiology is central in this. Imaging is used at the outset to estimate the probability of malignancy and degree of aggressiveness in malignant masses and to follow up masses for growth and morphologic change. Percutaneous biopsy is used to provide a more definitive histologic diagnosis and to guide treatment decisions, including whether active surveillance is appropriate. Emerging applications that may improve imaging assessment of renal masses include standardized assessment of cystic and solid masses and radiomic analysis. This article reviews the current and future role of radiology in the care of patients with renal masses undergoing active surveillance.