Introduction
The evaluation of renal masses is a frequent challenge in radiology. While various advanced imaging techniques such as contrast-enhanced ultrasound (US), dynamic computed tomography (CT), and magnetic resonance imaging (MRI) are utilized, differentiating benign from malignant exophytic renal masses remains complex. The Bosniak classification system aids in categorizing cystic renal lesions, but further evaluation, including biopsy, is often necessary for Bosniak category 3 or higher cysts. Therefore, a practical and straightforward diagnostic sign applicable in both US and CT is highly desirable to enhance diagnostic accuracy without resorting to complex and costly procedures.
Inspired by prior MRI research [6], which identified the angular interface sign, this study investigates its effectiveness in differentiating benign and malignant exophytic renal masses using US and CT. The angular interface sign describes the shape of the mass-kidney interface, categorized as either angular (pyramidal-triangular) or wide (rounded). This retrospective study aims to validate the angular interface sign in US and CT, offering a simpler, more accessible diagnostic tool.
Materials and Methods
Patients
This retrospective study analyzed data from patients with renal masses recorded in our image archive between January 2008 and June 2010. Initially, 86 cases with renal masses ≥2 cm were identified. Exclusion criteria included cases with innumerable, closely spaced lesions (n=4) that could distort interface sign interpretation, lesions <2 cm, non-exophytic masses from atrophic renal parenchyma, and cases without a definitive final diagnosis (n=11).
A total of 71 patients with exophytic renal masses ≥2 cm were included, examined prospectively for the angular interface sign using US (n=23), CT (n=21), or both (n=16). Among these, 56 had single lesions, and 15 had multiple lesions. In cases with multiple lesions, the dominant lesion was assessed. The final diagnoses were confirmed through MRI (n=23), typical radiological imaging (n=13), follow-up data (n=11), or biopsy (n=24). The study group comprised 27 women and 44 men, with a mean age of 47 years, and the mean renal mass diameter was 43 mm.
Technique and Image Analysis
CT scans were performed using a 16-detector multislice CT system (Somatom Sensation 16-slice CT), generating axial and reformatted coronal images. US examinations were conducted using a 7.5 MHz convex transducer (GE Logic 9 Ultrasound Imager) to acquire coronal and axial image planes of the exophytic renal lesions.
Two independent radiologists, blinded to diagnostic data, evaluated the images. They classified exophytic masses based on their interface with the renal parenchyma as either “wide interface” (rounded) or “angular interface” (pyramidal-triangular). A positive angular interface sign was recorded if observed in either coronal or horizontal images.
Statistical Analysis
Interobserver agreement was assessed using Kappa analysis. Statistical analysis was performed using 2×2 tables to calculate sensitivity, specificity, accuracy, positive predictive value, and negative predictive value for differentiating benign and malignant exophytic renal masses based on the angular interface sign.
Results
For cystic lesions, the angular interface sign was present in all but two Bosniak category 1 cases and in all but one Bosniak category 2-3 case.
In cystic lesions with solid components and purely solid lesions within the benign group, the angular interface sign was positive in all except three cases (vascular malformation, oncocytoma, and xanthogranulomatous pyelonephritis). Conversely, in the malignant group, the angular interface sign was observed in only two renal cell carcinoma (RCC) cases. Other primary or metastatic malignant lesions predominantly exhibited a wide interface sign. Figures 1-3 illustrate these interface characteristics in different renal lesions.
Interobserver agreement for the interface sign was almost perfect (Kappa: 0.97). Based on the average readings from both radiologists, the angular interface sign was not detected in 6 benign cases (false negative) and was positive in 2 malignant cases (false positive). Statistical analysis revealed that the angular interface sign achieved 87.5% sensitivity, 91% specificity, and 87% accuracy in differentiating malignant from benign exophytic renal masses. The accuracy was 85% for complicated cystic lesions and 65% for solid lesions.
Figure 1. Investigated findings of the interface relationship for three different cases which are variated lesions of the right kidney zone. (a) Simple exophytic cortical cyst, triangular interface; (b) Primary marginal zone lymphoma case of renal focus, wide interface; (c) Multiseptate, heterogeneous echoe cystic lesions; triangular interfaces at dominant cystic mass (dashed lines).
Figure 1. Investigated findings of the interface relationship for three different cases which are variated lesions of the right kidney zone. (a) Simple exophytic cortical cyst, triangular interface; (b) Primary marginal zone lymphoma case of renal focus, wide interface; (c) Multiseptate, heterogeneous echoe cystic lesions; triangular interfaces at dominant cystic mass (dashed lines).
Figure 1. Investigated findings of the interface relationship for three different cases which are variated lesions of the right kidney zone. (a) Simple exophytic cortical cyst, triangular interface; (b) Primary marginal zone lymphoma case of renal focus, wide interface; (c) Multiseptate, heterogeneous echoe cystic lesions; triangular interfaces at dominant cystic mass (dashed lines).
Figure 2. (a) Atypical angiomyolipoma; heterogeneous density lesion arising from the upper pole of the right kidney, coronal reformatted contrast-enhanced multidetector CT image. Triangular interface; (b) A case of papillary type RCC; axial view in the contrast-enhanced CT section, large interface (dashed lines).
Figure 2. (a) Atypical angiomyolipoma; heterogeneous density lesion arising from the upper pole of the right kidney, coronal reformatted contrast-enhanced multidetector CT image. Triangular interface; (b) A case of papillary type RCC; axial view in the contrast-enhanced CT section, large interface (dashed lines).
Figure 3. The reported cases that excisional biopsy compatible with incidental RCC of US appearance monitored as: (a) Echogenic; (b) Axial; and (c) Coronal, noteworthy heterogeneous enhancement on contrast-enhanced CT images. Renal interface relationship of the lesion is observed widely (dashed lines).
Figure 3. The reported cases that excisional biopsy compatible with incidental RCC of US appearance monitored as: (a) Echogenic; (b) Axial; and (c) Coronal, noteworthy heterogeneous enhancement on contrast-enhanced CT images. Renal interface relationship of the lesion is observed widely (dashed lines).
Figure 3. The reported cases that excisional biopsy compatible with incidental RCC of US appearance monitored as: (a) Echogenic; (b) Axial; and (c) Coronal, noteworthy heterogeneous enhancement on contrast-enhanced CT images. Renal interface relationship of the lesion is observed widely (dashed lines).
Discussion
Various imaging modalities, including US, CT, MRI, and PET-CT, are employed in the differential diagnosis of renal masses. Complex techniques like contrast US and dynamic-diffusion-perfusion imaging aim to characterize renal masses. However, definitive diagnosis with 100% accuracy remains elusive, particularly for non-contrast-enhancing lesions. Advanced techniques with multiple parameters are often required to differentiate benign and malignant lesions in these cases.
This study evaluated the angular interface sign in US and CT, previously validated in MRI [6], to determine if similar diagnostic discrimination could be achieved with simpler modalities. While interobserver variability can affect renal lesion diagnosis [7], our study demonstrated almost perfect interobserver agreement, likely due to the simplicity of the angular interface assessment. Traditional diagnostic approaches, even considering cystic components or lipomatous content suggestive of angiomyolipoma, can still misdiagnose RCC [8, 9], highlighting the need for more reliable benignity indicators.
Monitoring lesion growth rate is another diagnostic strategy, with malignant masses typically growing faster (0.3-0.5 cm/year). However, growth rate assessment is subjective and not always reliable due to benign lesions also exhibiting growth, and slow-growing malignancies. Furthermore, frequent CT follow-ups for growth monitoring expose patients to radiation and increase costs [10].
This study demonstrates that the angular interface sign, a simple morphological assessment, can be effectively used in US and CT to differentiate benign from malignant exophytic renal masses. Its ease of application and reproducibility offer a valuable alternative, especially in MRI-contraindicated or emergency situations. The high interobserver agreement and diagnostic accuracy (87%) suggest that evaluating the lesion-kidney interface type via US or CT can significantly aid in assessing lesion nature, particularly when other imaging characteristics are inconclusive.
The study’s primary limitation is the relatively small sample size, which limits subgroup analysis for cystic and solid lesions. Further research with larger cohorts is needed to validate the sign’s effectiveness, especially for Bosniak 3 lesions where management decisions are challenging. Another limitation is the requirement for masses to be ≥2 cm and exophytic. However, given that renal masses are often incidentally discovered at larger sizes and that collecting system neoplasia is less frequent, these limitations are not considered substantial. The angular interface sign offers a valuable advantage in non-contrast examinations, mitigating risks associated with iodine-based and MRI contrast agents, particularly in patients with renal insufficiency or contrast allergies [11].
Conclusion
The angular interface sign is a valuable tool for differentiating benign and malignant exophytic renal masses with high specificity (91%) and accuracy (87%) using US and CT. This simple, readily applicable sign can improve diagnostic confidence and potentially reduce the need for more complex and invasive procedures in evaluating exophytic renal masses.
REFERENCES
[1] – [11] (References from the original article would be listed here, maintaining the original numbering)
NOTES
*Conflict of interest disclosure: The authors declared no conflicts of interest.
