MRI, CT offer answers to renal mass queries

The widespread use of cross-sectional imaging techniques means that renal masses are now a common incidental finding. The term renal mass covers a diverse group of pathologic entities, including inflammatory, vascular, and benign tumors and neoplastic lesions. Most renal masses are simple cysts that can be characterized easily and require no treatment or follow-up. But approximately 25% to 40% of all renal cell carcinomas are diagnosed after the unexpected discovery of a renal mass.1 Around 85% of these renal cancers will be adenocarcinomas. Most of the others will be transitional cell carcinomas of the renal pelvis.

The main question when diagnosing an incidentally discovered renal mass is whether it is a lesion highly suggestive of malignancy and requiring surgery or a lesion requiring nonsurgical management and follow-up studies or image-guided percutaneous biopsy. Imaging findings interpreted in their proper clinical context should help differentiate between these entities (see table).

Determining the location and nature of the observed mass will be essential for a correct diagnosis. Masses adjacent to the kidney (pararenal lesions, splenic or hepatic masses, adrenal tumors, etc.) could all be mistaken for a renal mass. Infection, vascular lesions, and developmental anomalies could appear as pseudomasses. These possibilities must all be ruled out. Typical benign angiomiolipomas should be characterized, simple cysts and renal cystic tumors should be differentiated, and the type and grade of tumor in solid malignant masses should be identified.

High-quality imaging is essential when detecting and characterizing renal lesions. Multislice CT scanners, with their fast scanning and thin-slice acquisition capabilities, and advances in MRI technology have improved renal imaging significantly. Near-isotropic voxels offer the possibility of 3D images and high-resolution multiplanar reformatting (Figure 1).

Identifying a mass's precise location could influence decisions on management and surgery. Preoperative planning for nephron-sparing surgery also requires information about the tumor site, arterial and venous anatomy, and the relationship of the tumor to the pelvicaliceal system.2 Coronal and sagittal reformatted views are useful for identifying masses at the renal poles.

Developments in imaging technology have also improved our ability to detect small masses and to define their internal architecture and composition. Different scanning phases can be obtained by varying the acquisition time after contrast injection. The renal anatomy can now be depicted much more clearly in each phase using this approach. An unenhanced scan followed by a multiphasic contrast-enhanced examination is essential for detecting enhancing areas.

The corticomedullary phase (35 to 40 seconds after injection of contrast) has been proposed for vascular mapping and pseudotumor characterization. The nephrographic phase (90 to 100 seconds postcontrast) is critical for tumor detection and demonstration of enhanced areas in hypovascular tumors (Figure 2). Excretory-phase imaging could be needed if a transitional cell carcinoma is suspected.3 Image subtraction may be used to demonstrate enhancement in CT or MRI, though accurate image coregistration is mandatory.

SYMPTOMS AND SIGNS

Clinical manifestations are of great importance when it comes to determining the nature of an apparent renal mass. For example, clinical data may help differentiate an infection from a true mass. Radiological findings, such as ill-defined margins and perinephric stranding, may help confirm this diagnosis. Developmental abnormalities and anatomical variants should be kept in mind when faced with otherwise healthy patients. The patient's age should also be considered. Wilms' tumor, for instance, is the most common abdominal neoplasm in children aged one to four years, while renal cell carcinoma is exceptional in children.

Approximately 4% of renal cancers are associated with genetic disorders, such as von Hippel-Lindau disease, tuberous sclerosis, hereditary papillary renal cancer, Birt-Hogg-Dube syndrome, or hereditary renal oncocytoma (Figure 3).4 Hereditary renal cancers must be considered in young patients with renal malignancies, and in cases involving multiple renal tumors. Metastasis and lymphomatous infiltration must be excluded in patients with a known extrarenal primary malignancy or lymphoma. Both of these conditions commonly affect the kidney.

Characterization of a renal mass involves the evaluation of several features on imaging. The presence of fat will influence the differential diagnosis considerably. An unenhanced CT scan, and/or fat-suppressed and dual-echo T1-weighted gradient-echo (GRE) chemical-shift MRI will identify macroscopic or microscopic fat content. Angiomyolipoma is always proposed first in such cases, though CT will not visualize fat in 4.5% of angiomyolipomas. This diagnosis should still be considered if the renal mass shows high density on unenhanced CT, homogeneous enhancement on contrast-enhanced CT, low signal on T2-weighted MRI, and a loss of signal on opposed-phase double-echo T1-weighted GRE MRI.3-5

Calcium is a nonspecific sign associated with renal masses, but its presence could still be helpful. Between 7% and 18% of renal cell carcinomas will show calcification.6 Renal cell carcinoma is the most common calcified renal mass. An initial diagnosis of angiomyolipoma should not be considered if a lesion contains fat and calcium.3 The mass is more likely to be malignant in this case.

Renal cell carcinomas are usually hypervascular masses, although papillary or chromophobe renal cell carcinomas are frequently observed as hypovascular masses.6 Hypervascular lesions enhance after the injection of contrast material, so observed enhancement raises the likelihood of malignancy dramatically. A threshold of 20 HU is considered to be a clear indication of real enhancement. Pseudoenhancement (10 to 20 HU) must be excluded.3 When a mass is incidentally detected during a contrast-enhanced CT scan without a preliminary precontrast scan, de-enhancement (i.e., decrease in attenuation values at delayed postcontrast CT compared with those at initial enhancement) is an equally valid indicator of vascularity within the mass.7

There is no clear way of determining enhancement with MRI. Options include image subtraction techniques and percentage enhancement calculations from signal intensity. Less than 15% of relative enhancement is seen in benign masses. MRI is an alternative to CT in calcified masses. Calcium usually shows low signal intensity on T1-weighted MRI, making the enhancement more apparent in a calcified mass on gadolinium chelate-enhanced T1-weighted MRI than on contrast-enhanced CT.

The manner in which a renal mass involves the kidney can provide additional diagnostic clues. Renal cell carcinomas usually exhibit an expansive appearance with exophytic growth. Infiltrative lesions (transitional cell carcinomas, lymphoma, and infection) enlarge the kidney while maintaining the reniform contour and showing ill-defined zones of transition with the normal parenchyma.8

The epicenter of an infiltrative mass should also be evaluated. Renal sinus involvement usually predominates in transitional cell carcinoma and lymphoma, while parenchymal involvement is usually found in infiltrating renal cell carcinoma, metastases, and lymphoma. Perirenal involvement can be found in renal cell carcinomas, too, though it is more common in hemorrhage, infection, lymphoma, and metastasis.9 Multifocal and bilateral involvement is usually due to renal metastases, lymphoma, and hereditary renal cancers. Multiple tumors are more often seen in papillary renal cell carcinoma than in clear cell renal carcinoma.10

IMAGING-LED MANAGEMENT

Most renal masses, including simple cysts, inflammatory conditions, angiomyolipomas, and malignant masses, can be characterized accurately from radiological features. The main aim when approaching an incidental renal lesion is to establish whether surgery is required or not. Each patient's clinical history, age, and risk for surgery will influence the treatment choice.

Several general rules can be followed when considering solid renal masses. Follow-up is recommended for small angiomyolipomas, though they should be excised if larger than 4 cm in diameter owing to the high risk of bleeding.11 A direct surgical approach should be taken if clear signs of malignancy are present.12 Solid renal masses that cannot be diagnosed definitively as benign on imaging alone should undergo biopsy or simply be excised. Although solid renal masses in adults usually represent renal cell carcinomas, a significant number will be benign lesions that cannot be characterized preoperatively. This group comprises mainly oncocytomas and angiomyolipomas.

A direct correlation has been made between malignancy and lesion size: The smaller the renal mass, the greater the probability of it being benign.13 Size alone is not always a reliable criterion, though, when planning the management of a renal mass. One study showed that even among small renal cell carcinomas (up to 3 cm), there is a significant incidence of high nuclear grade tumor extension beyond the renal capsule as well as node metastases.14 The management of small solid renal masses consequently remains controversial. Some small solid cancers grow slowly or not at all, and these can be monitored on follow-up. A more aggressive approach has to be encouraged in high-risk situations, such as patients suspected of having hereditary renal cancer or a papillary tumor.4

Percutaneous biopsy can now provide detailed information on the histological features of renal lesions. Histochemical, inmunocytochemical, and cytogenic profiles of renal masses have been recognized.15 The sensitivity of biopsy for diagnosis of malignancy is 80% to 100%.15,16 Well-established indications for percutaneous biopsy include patients with known extrarenal primary cancer or lymphoma, radiological signs suggestive of unresectable renal cancer, comorbidity, and suspicion of infection.15

Cystic renal masses pose a different problem to solid renal masses. These masses can be divided into three categories: simple cysts, complicated cysts, and renal cystic cancers. Simple cysts will need no further follow-up. Complicated cysts and renal cystic cancers can demonstrate a wide spectrum of features on imaging. A cyst must not be considered simple if it shows calcification, enhancement, internal septations, nodularity, thick wall, multilocular pattern (more than three or four septations), hyperdense areas on unenhanced CT, or high signal on T1-weighted MRI. Malignant tumors (Figure 4), complicated cysts (infected or hemorrhagic), and abscesses may show some of these features.

Bosniak or Hartman classifications17,18 are helpful in determining the best approach to these cystic masses. These classifications are only clinical guides for deciding management, though, and not pathological classifications. A complete review of cystic renal cancers is beyond the scope of this article. As a general rule, the most worrisome imaging feature should be the one used to establish appropriate management.

Bosniak types I (simple cyst) and II (mildly complex cysts) are considered benign. Bosniak type IV lesions (cystic neoplasms) should always be surgically excised. Ten percent of all renal cell carcinomas are cystic.17 Cystic tumors usually have a slower growth rate and a better prognosis than solid masses.

Problems arise in differentiating between category II and III cysts because some category II cysts have predominantly benign but inconclusive imaging features. These features are not complicated enough to put the cyst in category III. For that reason, a category IIF (F = follow up) is used. Significant interobserver variability comes into play when separating Bosniak type IIF from type III (indeterminate cystic masses whose surgical resection is recommended in most instances). The risk of malignancy in a Bosniak IIF lesion is approximately 5%, whereas it is around 50% in a Bosniak type III lesion.18

Bosniak type III lesions continue to pose a diagnostic dilemma. This group has a high prevalence of benign lesions, suggesting a role for percutaneous biopsy. The sensitivity of biopsy is lessened in small renal masses and predominantly cystic lesions, however.16

A negative biopsy does not necessarily mean that malignancy can be excluded.15,16 The lack of a standard follow-up interval also makes it difficult to use clinical follow-up as a way of classifying cystic renal masses confidently as benign.

Preliminary data on the role of new MRI sequences such as diffusion-weighted imaging and diffusion tensor imaging for evaluating renal diseases are now available. Few studies have looked at apparent diffusion coefficient values for renal masses, however.19 Lesions requiring surgery tend to show significantly lower ADC values than lesions that do not require excision. Further investigations are required to assess possible differences in ADC value between different renal masses.

Information from imaging is important for the detection, characterization, and management of renal masses. Radiological features and patterns of involvement are basic tools for diagnosing renal lesions. CT is currently the modality of choice for diagnosing and staging renal neoplasms. MRI can be used as a problem-solving tool for the evaluation of renal anomalies. Imaging-guided percutaneous biopsy plays an important role when renal masses cannot be characterized accurately with imaging alone. New imaging techniques may perhaps one day help us to diagnose and characterize renal masses entirely noninvasively.

DR. FIGUEIRAS and DR. MARTIN are radiologists in the radiology department at the University of Santiago de Compostela hospitals (CHUS) in Santiago de Compostela, Spain. DR. ISIDRO is a radiologist in the radiology department at the Juan Canalejo Hospital in Coruna, Spain. Assisting in the preparation of this manuscript were Dr. Sandra Baleato Gonzalez, a resident radiologist, and Dr. Manuel Otero Echart, a radiologist, both in the radiology department at CHUS.

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• Kim JK, Kim SH, Jang YJ, et al. Renal angiomyolipoma with minimal fat: differentiation from other neoplasms at double-echo chemical shift FLASH MR imaging. Radiology 2006;239(1):174-180.

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