Diagnostic performance of contrast enhancement to differentiate benign and malignant renal lesions in CT and MRI: a systematic review and meta-analysis of diagnostic test accuracy (DTA) studies

OBJECTIVE

To perform a systematic review and meta-analysis of the diagnostic performance of contrast enhancement to differentiate benign and malignant renal lesions using CT and MRI.

MATERIAL AND METHODS

A systematic literature search of databases was performed between January 1, 1980 and September 26, 2022. We included studies reporting the accuracy of CE thresholds on CT and MRI indeterminate renal lesions, with pathologic examination and follow-up as the reference standard. Studies meeting the inclusion criteria underwent quality assessment with the Cochrane recommendation for diagnostic accuracy study Quality Assessment 2. We excluded studies with high risk of bias. Summary estimates of diagnostic performance were obtained with the bivariate Bayesian model for CT and MRI. Effects of different thresholds and index test modalities were investigated through subgroup analysis.

RESULTS

Eleven studies (1372 patients) using CT and six studies (218 patients) using MRI were included. Of the eleven studies, 15 parts from 9 studies were considered for the CT meta-analysis, and 6 parts from 3 studies for the MRI meta-analysis. Diagnostic performance meta-analysis on enhancement found a 96% summary sensitivity (95% CI 92, 98) and a 92% summary specificity (95% CI 85, 96) in 2056 renal lesions for CT; and 82% summary sensitivity (95% CI 65, 89) and an 89% summary specificity (95% CI 77, 95) in 634 lesions for MRI.

CONCLUSION

CT and MRI have high accuracy to determine enhancement and classify renal lesions, and both modalities can be used with confidence for this purpose. There are still some controversies about the optimal thresholds. Future research should evaluate outcomes and decision-making pathways to determine whether basing clinical decisions on a specific threshold on CT and MRI would do more harm than good.

Sporadic unilateral papillary renal cell carcinoma masquerading pyelonephritis: A case report

Hidden renal cell carcinoma (RCC) is not evident during severe inflammation such as acute pyelonephritis. We presented a 62-year-old male presenting with features of obstructive pyelonephritis managed with simple nephrectomy. However, the histopathology findings suggested unilateral papillary RCC. Our case showed that RCC might present with features of obstructive pyelonephritis.

Evaluation of class II cystic renal masses proposed in Bosniak classification version 2019: a systematic review of supporting evidence

PURPOSE

The Bosniak classification of cystic renal masses version 2019 (v.2019) includes an expanded number of types of masses in class II; such masses are considered benign in clinical practice. Data supporting these additions have not been well-documented. We aim to determine the proportion of malignant or probably malignant renal masses among the types added to Bosniak v.2019 class II.

METHODS

Multiple databases were searched for studies evaluating the proportion of malignant or probably malignant renal masses among new Bosniak v.2019 class II types, four for CT and two for MRI. Risk of bias and applicability was assessed using the QUADAS-2 tool.

RESULTS

Ten studies (2068 renal masses) met inclusion criteria. Among the four added class II types at CT, the proportion of malignancy among (1) 'homogeneous hyperattenuating (≥ 70 HU) masses at unenhanced CT' was 0% (0/32) in three studies; (2) 'homogeneous masses - 9 to 20 HU at unenhanced CT' was 0% (0/1454) in two studies, and (3) 'homogeneous masses 21 to 30 HU at portal-venous phase CT' was 0% (0/454) in four studies. Masses that are homogeneous, have low attenuation, and are too small to characterize on CT had no supportive evidence. Among the two added class II types at MRI, the proportion of malignancy among (1) 'homogeneous masses markedly hyperintense at unenhanced T2-weighted MR imaging (similar to CSF) was 0% (0/72) in one study, and (2) 'homogeneous masses markedly hyperintense at T1-weighted MR imaging (~ 2.5 × renal parenchyma signal intensity)' was 0% (0/32) and 5% (2/37) in two studies. Nine studies were at risk of bias within at least one QUADAS-2 domain.

CONCLUSION

The addition of six types of cystic renal masses to Class II in the Bosniak v.2019 proposal may be justified but based on limited evidence, with no evidence for 'homogeneous low attenuation masses that are too small to characterize' on CT, and thus considering them benign is in part based on expert opinion. Protocol Registration: PROSERO CRD42020196408.

The Incidental Renal Mass- Update on Characterization and Management

Renal masses are commonly encountered on cross-sectional imaging examinations performed for nonrenal indications. Although most can be dismissed as benign cysts, a subset will be either indeterminate or suspicious; in many cases, imaging cannot be used to reliably differentiate between benign and malignant masses. On-going research in defining characteristics of common renal masses on advanced imaging shows promise in offering solutions to this issue. A recent update of the Bosniak classification (used to categorize cystic renal masses) was proposed with the goals of decreasing imaging follow-up in likely benign cystic masses, and therefore avoiding unnecessary surgical resection of such masses.

Clinical Importance of Incidental Homogeneous Renal Masses That Measure 10-40 mm and 21-39 HU at Portal Venous Phase CT: A 12-Institution Retrospective Cohort Study

BACKGROUND. Incidental homogeneous renal masses are frequently encountered at portal venous phase CT. The American College of Radiology Incidental Findings Committee's white paper on renal masses recommends additional imaging for incidental homogeneous renal masses greater than 20 HU, but single-center data and the Bosniak classification version 2019 suggest the optimal attenuation threshold for detecting solid masses should be higher. OBJECTIVE. The purpose of this article is to determine the clinical importance of small (10-40 mm) incidentally detected homogeneous renal masses measuring 21-39 HU at portal venous phase CT. METHODS. We performed a 12-institution retrospective cohort study of adult patients who underwent portal venous phase CT for a nonrenal indication. The date of the first CT at each institution ranged from January 1, 2008, to January 1, 2014. Consecutive reports from 12,167 portal venous phase CT examinations were evaluated. Images were reviewed for 4529 CT examinations whose report described a focal renal mass. Eligible masses were 10-40 mm, well-defined, subjectively homogeneous, and 21-39 HU. Of these, masses that were shown to be solid without macroscopic fat; classified as Bosniak IIF, III, or IV; or confirmed to be malignant were considered clinically important. The reference standard was renal mass protocol CT or MRI, ultrasound of definitively benign cysts or solid masses, single-phase contrast-enhanced CT or unenhanced MRI showing no growth or morphologic change for 5 years or more, or clinical follow-up 5 years or greater. A reference standard was available for 346 masses in 300 patients. The 95% CIs were calculated using the binomial exact method. RESULTS. Eligible masses were identified in 4.2% of patients (514/12,167; 95% CI, 3.9-4.6%). Of 346 masses with a reference standard, none were clinically important (0%; 95% CI, 0-0.9%). Mean mass size was 17 mm; 72% (248/346) measured 21-30 HU, and 28% (98/346) measured 31-39 HU. CONCLUSION. Incidental small homogeneous renal masses measuring 21-39 HU at portal venous phase CT are common and highly likely benign. CLINICAL IMPACT. The change in attenuation threshold signifying the need for additional imaging from greater than 20 HU to greater than 30 HU proposed by the Bosniak classification version 2019 is supported.

Mimics and Pitfalls in Renal Imaging

There are several potential pitfalls that radiologists face when interpreting images of the kidneys. Some result from image acquisition and can arise from the imaging equipment or imaging technique, whereas others are patient related. Another category of pitfalls relates to image interpretation. Some difficulties stem from methods to detect enhancement after contrast administration, whereas others are benign entities that can mimic a renal tumor. Finally, interpretation and diagnosis of fat-containing renal masses may be tricky due to the complexities discerning the pattern of fat within a mass and how that translates to an accurate diagnosis.