MR imaging of indeterminate renal lesions

Once technically acceptable images are acquired, the first step in analysis of any renal mass requires classifying the lesion as cystic or solid. A lesion with generally uniform hyperintensity on T2W sequences, similar to cerebral spinal fluid or gallbladder, can be confidently labeled “cystic.” The most common etiologies of a cystic renal lesion include simple cyst, complicated benign cyst, and cystic RCC. Internal hemorrhage or proteinaceous debris within an indeterminate cystic lesion may result in relative T2 hypointensity compared with a simple cyst and should correspond to T1 hyperintensity on the fat-saturated T1W sequences. Measurement of signal intensity on the pre- and postcontrast sequences or the application of subtraction software will assess whether a lesion is truly cystic or merely a T2 hyperintense solid mass (Figure 1). If there are no dominant thickened septations or nodular components, a diagnosis of benign cyst (simple or hemorrhagic) can be assigned.

The Bosniak CT criteria for cystic renal classification have been widely adapted for MRI interpretation.⁵ Unfortunately, compared with its appearance on CT, cyst wall calcification is not well appreciated on MR images. A small number (<4) of thin “hairline” septations within a cyst without other concerning features is consistent with a benign diagnosis. Lesions with ≥4 septations should be followed with additional imaging examinations (Bosniak Class II or IIF). Any thickened or irregular septations or nodular enhancing components should raise the specter of a cystic renal cell carcinoma and require histologic sampling (Class III or IV) or interval follow-up in the frail patient (Figure 2 and Figures 5 through 8).

If follow-up imaging is recommended, consider imaging beyond the routine 2-year criteria for benignity because of the potentially long doubling time of RCC. Comparison with prior imaging examinations is helpful in documenting lesion stability. The “water” within benign cysts should be unrestricted and should not appear hyperintense on DWI sequences. Although it is not sufficiently specific to exclude a cystic RCC, DWI is used to help triage cystic lesions with borderline wall or septal thickening to follow-up imaging versus pathologic sampling. A rare benign cystic lesion, the multilocular cystic nephroma (MLCN) presents as a mass lesion with multiple internal cysts. The classic MLCN patient demographic includes either a preteen boy or a middle-aged woman. Multilocular cystic nephroma may herniate into the central renal medulla/renal pelvis, allowing for confident diagnosis (Figure 9). Nevertheless, as with the other cystic lesions, thickened septa or enhancing nodularity in MLCN should triage the patient to pathologic sampling or surgical excision.

The differential diagnosis for a solid enhancing renal mass incorporates a wide range of malignant and benign entities. Angiomyolipoma (AML) and oncocytoma represent the most common benign lesions, while RCC, renal metastasis, and transitional cell carcinoma top the list for malignant masses. As its name implies, an AML is composed of vascular, smooth muscle and lipid elements. The benign AML can be seen in isolation or can be associated with tuberous sclerosis, in which case there are often multiple and bilateral renal lesions.

Reflecting the various internal histologic components, marked variability typifies the MR appearance of an AML, including heterogeneous T2 hyperintensity and variable enhancement on dynamic contrast-enhanced sequences. The superior tissue characterization of MRI allows more precise identification of the fat component within an AML than does either CT or US. The classic imaging characterization of AML requires identification of focal areas of lipid. Colloquially, these fatty foci are termed macroscopic fat because they are appreciable to the naked eye and, on imaging, appear similar to the relatively pure fat seen in the subcutaneous tissues or perinephric space.

Sonographically, fat within an AML shows focal hyperechogenicity. On a CT scan, focal areas of fat density (<0 HU) are seen within AMLs. Chemically selective fat-saturation pulses are incorporated into both T1W and T2W sequences for similar detection of macroscopic fat (Figure 3). The great majority of AMLs contain foci of macroscopic fat, which are best detected on T1W images acquired with and without fat saturation and with all other major parameters held constant.

In a smaller subset of AMLs, the intralesional fat is more homogeneously dispersed throughout the tumor. In these lesions, the fat can be detected using a dual-phase gradient-echo T1W sequence with in- and opposed-phase images. The presence of signal loss within the renal lesion on the opposed-phase versus the in-phase acquisition confirms the presence of intravoxel fat (Figure 4). Some authors label this lipid-poor AML, although pathologically intralesional fat is still seen microscopically. A signal loss <40% on opposed-phase GRE suggests the diagnosis of lipid-poor AML. A subset of RCC can also contain intralesional fat; however, the degree of signal loss on opposed-phase images is typically <20%.¹⁰ On opposed-phase GRE T1W images, the classic AML macroscopic fat can be identified by a hypointense “india ink” outlining artifact separating the fatty component of the lesion from the surrounding water-containing elements. This occurs due to signal loss in those voxels at the interface of fat and water-based tissues.

Oncocytoma, the most common benign tumor of the kidney, is an adenoma arising from type B intercalated cells of the cortical collecting duct. On MRI, the oncocytoma is usually a well-circumscribed mass that is T1 hypo- or isotense relative to the renal cortex and exhibits variable T2 heterogeneity. Occasionally present in larger lesions (>5 cm), a T1 hypointense, T2 hyperintense, and hypoenhancing central scar can suggest the diagnosis (Figure 10). In smaller lesions, a homogeneous persistent enhancement pattern can suggest the diagnosis, but is not definitive. The classic angiographic “spoke-wheel” pattern of oncocytoma vessels is rarely seen on MRI. In comparison with AML, no definitive MRI findings can differentiate oncocytoma from RCC. For many years, the identification of a solid renal mass without visible macroscopic fat triaged the patient to surgical resection. Previously, pathologic analysis could not distinguish a benign oncocytoma from a chromophobe subtype of RCC since oncocytic cells are seen in both lesions. In a large retrospective review, 13% of resected solid renal masses were found to be benign, and this increased to >25% for lesions smaller than 3 cm.¹¹ Immunohistochemical and cytogenetic analyses have now improved to allow for differentiation between these benign and malignant entities.^12,13 With the rise in detection of smaller incidental lesions, percutaneous biopsy with pathologic analysis can play a significant role in avoiding unnecessary partial or complete nephrectomy for benign lesions.¹⁴ In an increasing number of institutions, when a lesion remains indeterminate following renal protocol MRI or CT, a percutaneous biopsy is now the standard of care. Renal mass biopsy should particularly be considered for small (<2 cm) lesions with persistent uniform enhancement and in patients who are poor candidates for surgical resection.

Renal cell carcinomas comprise the majority (approximately 80%) of primary renal neoplasms and 2% of all malignant tumors.¹⁵ Renal cell carcinoma arises from the tubular epithelium of the kidney and can be divided into several histologic subtypes: clear cell RCC (most common, ~75%), papillary (~15%), chromophobic, oncocytic, and collecting duct. Identification of the sub-type does not have reliable prognostic significance,¹⁶ but does account for the variable imaging appearance of RCC. In general, the solid varieties of RCC lesions are well-circumscribed, cortically based, heterogeneously T2 hyperintense, and isointense on noncontrast T1W sequences, and exhibit avid arterial-phase enhancement.

In some cases, the histologic subtype can be suggested by specific imaging features. For example, cystic change is more closely associated with the papillary subtype (Figures 6 through 9). Clear cell RCC are usually solid masses and may show mild signal loss on opposed-phase gradient imaging, consistent with a component of intravoxel fat. Occasionally, a solid RCC will not show typical arterial enhancement, and subtraction images can be helpful in identifying subtle tumoral enhancement (Figure 11). The Bosniak criteria apply only to lesions that are predominantly cystic. As with cystic lesions, findings of restricted diffusion within a solid renal lesion on DWI sequences is suggestive of malignancy. Nevertheless, this application is not sufficiently sensitive to confidently categorize a lesion as benign in the absence of DWI restriction. Once RCC is suspected, an assessment of renal vein invasion and local lymphadenopathy will impact the tumor TMN stage. Occasionally, RCC may arise closer to the medulla of the kidney and can be confused with transitional cell carcinoma (TCC). Calyceal, renal pelvis, or ureteral distortion, thickening, or enhancement is more frequently seen with TCC. Clinically, TCC presents with hematuria more often than does RCC. Angiomyolipoma, oncocytoma, and RCC are not the only solid renal lesions encountered by radiologic imaging. More rare entities include fibroma, leiomyoma, extramedullary hemato poiesis, hemangioma, and juxtaglomerular cell tumors; however, there are no definitive imaging features of these entities that would obviate pathologic diagnosis. Additionally, the kidneys are a relatively common site for metastases from numerous cancers. An isolated renal metastasis is rare; additional extrarenal deposits are usually present, making metastatic disease less of a diagnostic dilemma.