Introduction

Diagnostic radiologists translate imaging data into words that influence patient care [1]. These words must be precise and accurate to optimize communication between radiologists, referring providers, and patients [2,3,4,5,6,7,8,9,10]. Emerging data indicate the need for structured reports [11,12,13,14,15] that use specific well-defined terms [16,17,18,19,20,21] to improve clarity and understandability. To address these goals, schemas such as the Breast Imaging Reporting and Data System (BI-RADS) [2, 3] have created standardized lexicons that define terms to be used when radiologists are describing imaging findings, either in a clinical radiology report or in a research study. Such lexicons define terms and provide formal recommendations about which specific terms should be used or avoided [12, 13].

No such lexicon exists for CT and MRI of renal masses. Terminology used in radiology reports and research studies remains highly variable [22,23,24,25]. For example, numerous terms have been used to describe the presence of ‘fat’ in a renal mass, including terms that refer to fat cells (e.g., ‘macroscopic fat,’ ‘macroscopic lipid,’ ‘extracellular fat,’ ‘bulk fat’) and terms that refer to both fat cells and other cells that contain fat (e.g., ‘microscopic fat,’ ‘intracellular fat,’ ‘intracellular lipid,’ ‘intracytoplasmic fat,’ ‘intra-voxel fat’) [26,27,28,29]. Terms used inconsistently or inaccurately, or that are not based on explicit definitions, lead to confusing or ambiguous clinical reports [29], interobserver variability, and difficulty interpreting and reproducing research studies [23,24,25].

There is a need to develop standard terms for imaging features of renal masses that will promote accurate and consistent reporting in both clinical practice and research. The purpose of this study was to develop a standardized lexicon and image atlas to describe renal masses at CT and MRI.

Methods

This study was undertaken by the Society of Abdominal Radiology (SAR) Disease-Focused Panel (DFP) on renal cell carcinoma (RCC). Only terms relevant to CT and MRI were included because CT and MRI are the most common modalities used to evaluate indeterminate renal masses. Terms describing imaging protocols (e.g., the phases of enhancement after intravenous contrast material administration), anatomic structures (e.g., renal cortex), and specific disease entities (e.g., renal cell carcinoma) were not evaluated because they are outside the scope of this project and definitions for these terms and concepts exist throughout the literature. In addition, some terms that are specific to cystic masses (e.g., ‘nodule,’ ‘septa’ and the adjectives used to describe them) were excluded from this project because they were being fully defined as part of a parallel effort to update the Bosniak classification [30]. All 13 radiologist members of the RCC DFP from nine academic institutions in the US and Canada participated in the study. All participants considered renal masses a primary area of clinical and research interest.

Study design: modified Delphi method

This multi-institutional, prospective, quality improvement project was exempt from IRB oversight, and utilized a modified Delphi method [31,32,33,34]. One radiologist member of the panel (AS), with the help of a radiology clinical fellow (HP) served as the ‘coordinator’ who compiled the initial list of terms, prepared the questionnaires, and collected and analyzed the data. The remaining twelve radiologist members of the RCC DFP were invited to participate in the creation of the lexicon. All members are fellowship-trained abdominal radiologists with mean 13 years of experience as attending radiologist (range 5–30 years). Figure 1 outlines the process that was followed, and Appendix 1 provides the detailed methodology. Three rounds of questionnaires and one teleconference (after the second round) were conducted to build consensus (Appendix 1). Following the third round of questionnaires, the manuscript was created. Blinded edits to the manuscript were made and final consensus was reached. Based on prior literature, ≥ 80% agreement at the end of three rounds was considered sufficient ‘consensus’ regarding the inclusion or exclusion of each term and its definition [35,36,37,38]. Individual panelist responses remained anonymous in all three rounds. After the lexicon was finalized, official endorsement was obtained from the SAR Board of Directors.

Fig. 1
figure 1

Flowchart demonstrating the modified Delphi process used to create a lexicon of terms for the description of renal masses at CT and MRI

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Results

The Round 1 questionnaire was sent to 12 panelists by one coordinator; 11 completed it. Of the 11 panelists who participated in Round 1, one was not able to participate further; as a result, ten panelists completed the questionnaires in Rounds 2 and 3.

A total of 35 terms were included in the first round (Table 1). There were 17 basic imaging terms, eight CT terms, six MRI terms, and four miscellaneous terms. Two additional terms were proposed in the first round (‘magnetic susceptibility’ and ‘growth rate’). By the end of the third round, of the 37 terms, 31 had ≥ 80% agreement to remain in the lexicon (Table 2) and six had ≥ 80% agreement to be removed from the lexicon (Table 3). Of the 31 included terms, 29 had 100% agreement to include and two had 90% agreement to include. Of the six excluded terms, four had 100% agreement to exclude, one had 90% agreement to exclude, and one had 80% agreement to exclude (Table 3).

Table 1 Initial list of imaging terms evaluated during the first round of a modified Delphi process to create a lexicon of standard terms to be used in the description of renal masses at CT and MRI
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Table 2 Final consensus (≥ 80% agreement) terms and definitions to be used in the description of renal masses at CT and MRI
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Table 3 Excluded terms and level of agreement for exclusion
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Of 31 included terms, consensus on definition was achieved for 30 (13/14 basic imaging terms, 8/8 CT terms, 6/6 MRI terms, and 3/3 miscellaneous terms). Of these, 17 terms (10 basic imaging terms, four CT terms, and three MRI terms) had 100% agreement, 11 terms (two basic imaging terms, four CT terms, two MRI terms, and three miscellaneous terms) had 90% agreement, and two terms (one basic imaging term and one MRI term) had 80% agreement. Consensus was not reached for the definition of one general term (solid mass, 60% agreement). Table 2 details the proposed definitions and points of disagreement. The lexicon was endorsed by the SAR Board of Directors.

Discussion

We developed a consensus-based lexicon to describe renal masses at CT and MRI that addresses a knowledge gap in clinical radiology reporting. As radiology practice becomes more value-based, there is an increasing emphasis on the quality and actionability of radiology reports. This renal mass lexicon may make radiology reports of renal masses more standardized, actionable, and easy to understand for referring providers, patients, and radiologists, and may improve the reproducibility and generalizability of related research. Further, a similar lexicon may be necessary in other clinical contexts; we also hope that this study provides a guide for similar such efforts in the future.

This lexicon addresses clinical reporting as well as the imaging-based research of renal masses. The terms and their definitions address a variety of imaging-based features which add clarity to radiology reports and renal mass imaging research. Some specific potential benefits include distinguishing between macroscopic and microscopic fat at MRI [26,27,28,29]. These terms reflect challenging concepts and are critical to the diagnosis of masses which contain fat cells or other cells that contain fat in their cytoplasm [39]. The lexicon also addresses imaging features used to differentiate solid and cystic masses, including the presence of intravenous contrast material enhancement [40,41,42]; these are important considerations when applying the Bosniak classification [30]. The lexicon does not include some of the terms used to describe cystic renal masses as the recent update proposal to the Bosniak classification provided a comprehensive description of features and definition of terms [30]. Use of the lexicon may promote consistent terminology, decrease interobserver variability, and improve reproducibility—issues that the Bosniak update proposal aimed to improve [30]. This lexicon uses consistent terminology as in the Bosniak update, but expands it to encompass all terminology related to renal masses. The lexicon also adds clarity to the description of renal mass location and growth pattern; both have implications for prognosis and management, particularly surgical planning and calculating the R.E.N.A.L nephrometry score [43].

When conducting research, investigators from different institutions do not always analyze the same imaging features. When they do, the exact terms and their definitions are either incompletely expressed or inconsistent with prior studies [22,23,24,25]. For example, in 2014 and 2015, Karlo et al. and Shinagare et al. each reported on the radiogenomics of renal cell carcinoma [23, 24]. While five of six mutations (VHL, BAP1, PBRM1, SETD2, KDM5C, MUC4) studied in these reports were the same, of the 10 imaging features evaluated, only four were evaluated in both studies. Furthermore, the conclusions reached for the four shared imaging features also differed despite that both studies utilized similar study cohorts; this was thought to be due to differences in the definitions of imaging features used in these studies [23, 24]. For some of the imaging features studied, the definitions were unclear. For example, Shinagare et al. defined ‘necrosis’ as 'hypodense, non-enhancing areas which were not sharply demarcated and lacked apparent walls.' Karlo et al. defined ‘necrosis’ as “presence‘or absence of areas within the tumor that did not demonstrate contrast enhancement during the nephrographic and delayed phases' [23, 24]. These studies also differed in whether specific definitions were provided for well-defined margin and tumor architecture [23, 24]. This is not meant to be a criticism of either study; indeed, during our Delphi method, there was no initial consensus for the imaging-based definition of ‘necrosis.’ Although consensus was eventually reached, the panel determined that necrosis cannot be diagnosed with certainty at imaging and therefore was difficult to define using imaging (Table 2). These difficulties highlight the need for a lexicon of standard terminology.

The modified Delphi process we used offered several advantages for arriving at expert consensus [31,32,33,34, 38]. Since each panelist completed the questionnaire independently and anonymously, their opinions were expressed freely and with sufficient time to research and formulate views, as opposed to the in-person roundtable method where a few members can dominate the discussion, and others may not feel as confident or comfortable expressing their opinions [31,32,33,34, 38]. We shared individual anonymous opinions and opposing views in Rounds 2 and 3, and geographically disparate members from different time zones discussed the sources of disagreement during the teleconference.

Given that the definitions were created using expert consensus, some of the features were by necessity arbitrarily defined (e.g., ‘well-defined’). We were forced to rely on expert experience in the absence of published data or published definitions. There was initial disagreement on the definition of some terms, including basic terminology that is used in daily practice. ‘Renal mass’ was one of them. The panel eventually agreed that a renal mass should include any space-occupying abnormality as opposed to including only cysts and neoplasms. The panel believed that using a broad term that encompasses any pathology would decrease the chance of a non-neoplastic condition such as focal bacterial pyelonephritis from being misdiagnosed as a neoplasm. Both ‘cystic’ and ‘solid’ were also difficult to define. Although discriminating cystic masses from solid masses is a fundamental tenant in renal mass evaluation, at the time this work began, there were no established imaging criteria for doing so. This likely explains the difficulty in reaching consensus for the definition of a ‘solid’ mass. Overall, there was general agreement that a renal mass is considered solid when it is composed of ≥ 25% enhancing components or fat [30]; however, there was disagreement about what constitutes solid tissue. While all the panelists agreed that a solid mass may be comprised of enhancing tissue or fat, four of 10 panelists also commented that solid tissue does not always enhance and may appear as heterogeneous non-enhancing tissue at either CT or MRI.

‘Margin’ of a renal mass is often a source of confusion. The panel agreed that a ‘well-defined margin’ denotes a sharp, pencil-thin demarcation with an abrupt transition between the mass and the surrounding tissue encompassing at least 90% of the circumference of the mass. If a well-defined margin is not present, it is termed an ‘ill-defined margin.’ There also may be confusion about the internal reference standard to assess ‘CT attenuation,’ ‘MRI signal intensity,’ and ‘enhancement.’ There was consensus to use normal renal cortex as an internal reference standard. When a renal mass is heterogeneous, one or more regions of interest may need to be placed for accurate assessment. In prior literature, several terms are variably and interchangeably used to describe the presence of fat within a renal mass [26,27,28,29]. The panel recommends use of ‘macroscopic fat’ and ‘microscopic fat,’ and provides definitions for each [39].

The lexicon does not dictate when each term should be used. The terms in the lexicon complement the structured reporting template previously derived by the SAR RCC DFP [44,45,46]. These studies addressed what is preferred to be included in structured reports, such as mass type (cystic mass according to Bosniak classification vs. a solid mass), presence of fat, presence of enhancement, and radiologic stage—all considered ‘core’ features when reporting indeterminate renal masses [46]. This lexicon complements the prior work and provides definitions for various imaging terms that would help standardize the use of terminology in clinical reports and research studies.

The modified Delphi method has some inherent limitations, including its reliance on expert opinion informed by evidence, possible bias in selecting experts, dependence on questionnaire design, and ability of the coordinator to effectively compile the data. We tried to minimize the dependence on questionnaire design by the coordinator by allowing the panelists to propose additional imaging terms. The reliance on the ability of the coordinator to compile data was offset by using three voting rounds and a teleconference, as well as by allowing all authors to propose any final edits to the definitions. The panel included diverse members of the SAR RCC DFP with specific expertise in renal mass imaging, representing a broad range of academic experience. We did not attempt to define terms of specific disease entities (e.g., renal cell carcinoma, angiomyolipoma); these are defined elsewhere in the literature. We only included terms used at CT and MRI as these are the most common modalities used to evaluate indeterminate renal masses. An updated lexicon that includes other modalities (e.g., ultrasound) is planned.

The Radiological Society of North American (RSNA) endorses RadLex as a lexicon designed to achieve similar goals of consistency and reproducibility for all radiology reporting [4, 47]. We did not specifically incorporate terms defined by RSNA RadLex. While there is some common ground between RadLex and this lexicon, RadLex does not contain all the terms relevant to renal mass evaluation (e.g., ‘renal mass’ or ‘growth pattern’). Additionally, some of the terms used in RadLex are not appropriate for evaluation of a renal mass (e.g., instead of ‘well-defined margin’ and ‘ill-defined margin,’ which are germane to renal mass assessment, RadLex uses ‘smooth,’ ‘lobulated’ and ‘irregular,’ which are less relevant). The panel excluded from consideration several terms because they were either confusing, non-contributory, or lacking sufficient evidence. The term ‘renal capsule’ is an anatomic term, and hence outside the scope of this lexicon. The term ‘outline’ was excluded because the panel considered ‘margin’ to represent the same feature. Terms such as calcification at MRI, central scar, and rate of enhancement were excluded because either the clinical significance of these terms was unclear, or there was not enough data to define them. Finally, terms related to texture analysis were not included because the techniques for conducting this type of analysis are not standardized, data on accuracy and reproducibility are scant, and its clinical utility is uncertain. The members of the SAR RCC DFP acknowledge that some of these terms may need to be reevaluated for potential inclusion in the lexicon in the future as new data emerge about their importance in the management of renal masses. Finally, despite our efforts to standardize terminology, there may be persistent reader-level variability in the assessment of a renal mass [24]. We hope the specific definitions included in the lexicon will reduce this variability and take us a step closer to standardizing both radiology reports and research. Whether these goals are accomplished is a topic of further research.

In summary, this SAR-endorsed lexicon for the description of renal masses at CT and MRI has been created that attempts to address the inconsistencies and ambiguities that currently exist in clinical radiology reports and research related to renal masses. The process we used to create a renal mass lexicon may serve as a guide for the creation of lexicons in other imaging settings.