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Renal Masses
Detection of renal masses should be a high-priority task for radiologists examining the abdomen. Despite a great deal of research and many innovations in the treatment of renal cell carcinoma (RCC), the disease remains resistant to radiation therapy and chemotherapy. Modifiers of host biologic response, including agents such as interferon and interleukin, and newer biologic agents such as tyrosine kinase inhibitors have been studied extensively for the treatment of advanced RCC. However, therapeutic techniques have led to little improvement in the prognosis for patients with metastatic RCC. The 5-year survival rate of patients with metastatic RCC is only 5% to 10%. In addition, the incidence of RCC increased steadily by 38% between 1974 and 1990. Although little progress has been made in the therapy for RCC, the 5-year survival rate for patients with RCC has improved significantly—from 37% in the early 1960s to 65% for those diagnosed in 2005. Both of these trends (i.e., the increased incidence and the improved survival) appear to be due to improvements in the radiologic diagnosis of RCC that allow for the diagnosis to be made at an earlier stage of the disease. Low-stage RCCs can be successfully treated with surgery and ablation techniques. In addition, the prognosis for patients with low-stage disease at diagnosis is considerably more favorable than for those with a more advanced stage at diagnosis. Lead time and length biases may play some role in the apparent survival benefits, but clinical data strongly suggest improved outcomes due to earlier diagnosis of RCCs.
Renal masses may be detected, often incidentally, with a variety of imaging modalities. Most renal tumors are diagnosed incidentally. Approximately 75% of all RCCs are diagnosed after the incidental detection of a renal mass. Before the widespread use of cross-sectional imaging techniques, including sonography, computed tomography (CT), and magnetic resonance imaging (MRI), the rate of diagnosis was only 10%. The increase in incidental detection of renal masses is not surprising given the increasing use of cross-sectional imaging techniques. In a study that analyzed autopsies performed between 1958 and 1969, two thirds of RCCs occurred in patients in whom the diagnosis was not clinically suspected. Several studies have confirmed that there is an increase in the number of RCC cases detected during imaging studies performed for nonurological reasons. Patients with incidentally discovered RCC have a more favorable prognosis than do patients who present with urologic symptoms attributable to RCC. The classic triad of clinical findings, which includes flank pain, a palpable flank mass, and hematuria, was a common presentation for patients with RCC before the widespread use of cross-sectional imaging. However, nowadays this clinical triad is rarely seen, and its presence usually indicates the presence of advanced, inoperable disease. The difference in prognosis reflects the fact that most incidentally detected RCCs are either stage T 1 or stage T 2 , resulting in a much more favorable outlook than that of patients with advanced disease. In fact, the clinical course and disease stage in symptomatic patients have changed very little during the last 40 years. The dismal prognosis for RCC patients with advanced-stage disease makes detection of low-stage RCCs, often incidental findings, all the more important.
Therefore it can be said that substantial progress has been made with regard to improving the survival rate for patients with RCC. Much of this improvement can be attributed to early radiologic diagnosis of renal malignancies that in turn has resulted in a higher proportion of tumors that can be cured with surgical resection. For this reason, the detection and accurate diagnosis of renal masses are important tasks for radiologists.
This chapter describes the radiographic clues that are useful in the diagnosis of renal masses. Further, once detected, renal masses should be characterized and, if necessary, staged for proper management. Although this task may seem daunting at first, a few basic principles can be used to assist in the diagnosis of most renal masses.
Balls versus Beans
One concept that is very helpful in detecting and classifying renal masses is the basic shape of the mass ( Boxes 3-1 and 3-2 ). Most renal masses grow by expansion. These renal masses are usually spherical or ball shaped and they displace and compress, rather than infiltrate and invade, normal structures ( Fig. 3-1 ). As these masses enlarge, they expand from the normal parenchymal margins, either peripherally to the kidney or into the renal sinus, depending on the epicenter and primary direction of growth. Alternatively, some renal lesions grow by infiltration, a second pattern. These lesions grow along the latticework of the normal renal parenchyma. Although infiltrating lesions may swell the area of involved parenchyma, they do not greatly deform the shape of the kidney ( Fig. 3-2 ). The kidney retains its bean shape, and these infiltrative lesions are often referred to as beans, in distinction to the previously described balls. Beans are often more difficult to detect radiologically than balls because they are associated with little mass effect.
Box 3-1
Expansile Renal Masses
-
Radiographic Characteristics
-
Ball shaped
-
Approximately spherical
-
Expansile
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Often exophytic
-
Displace normal structures
-
Well-demarcated margins
Box 3-2
Infiltrative Renal Masses
-
Radiographic Characteristics
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Maintain bean shape of the kidney
-
Infiltrate normal structures
-
Poorly demarcated margins
In addition, both ball- and bean-shaped lesions may be solitary or multiple and the number of lesions is often helpful in determining the correct diagnosis. Boxes 3-3 and 3-4 list lesions in each of these two categories (i.e., balls and beans). The ball category includes most of the common renal masses, such as simple cysts, RCC, angiomyolipoma (AML), most metastases, oncocytoma, and abscesses. Alternatively, with geographic infiltrating lesions one should also consider the following three I s:
- 1.
I nfiltrating neoplasms
- 2.
I nflammatory lesions
- 3.
I nfarction.
Box 3-3
Solitary Expansile Renal Masses (Balls)
Box 3-4
Infiltrating Renal Masses (Beans)
Infiltrating neoplasms include transitional cell carcinoma (TCC) and squamous cell carcinoma (SCC), which spread from the urothelium to the renal parenchyma. Nowadays, these two subtypes of urothelial tumors are often classified by pathologists as urothelial carcinomas because the tumors often contain elements of each subtype. Uncommon infiltrating tumors include infiltrating RCC, the recently described renal medullary carcinoma, some metastases, leukemia, and lymphoma. All of these lesions are described in greater detail later.
Detection
Previous studies have shown that the sensitivity for detecting renal masses varies with different imaging modalities. The sensitivity for excretory urography is 67%, 79% for sonography, and 94% for conventional, nonhelical CT. The sensitivity of detection of renal tumors with MRI and multidetector CT is close to 100%. Furthermore, excretory urography lacks sufficient specificity for accurately characterizing any renal masses as benign . Therefore every renal mass detected with or suggested by excretory urography must be imaged with another technique.
Plain Films
Commonly, the first hint of a renal mass may be found on an abdominal radiograph. Renal masses may be visible on a radiograph or a tomogram of the abdomen, usually as ball-shaped masses extending from the kidney. Uncommonly, fat within the mass may increase its conspicuity. The presence of fat density within a renal mass is virtually diagnostic of an AML. Confirmation of intratumoral fat should be obtained with either CT or MRI before making a diagnosis of AML, a benign tumor. More commonly, calcifications are detected within a renal mass ( Fig. 3-3 ). Calcification within a renal mass is worrisome. Before the refinement of cross-sectional imaging techniques, a urology rule of thumb was a calcified renal mass is a surgical renal mass. This tenet remains true for many calcified renal masses even today. Although cross-sectional imaging is required to better characterize and guide management of a calcified renal mass, plain-film findings often provide significant information on the etiology of these masses.
When calcification is detected, its pattern should be scrutinized ( Box 3-5 ). A thin, peripheral rim of calcification most commonly occurs within the wall of a benign cyst ( Fig. 3-4 ). Although only 1% of cysts contain calcium, renal cysts are ubiquitous. Unfortunately, peripheral rim calcifications can also develop in renal neoplasms, especially in cystic RCCs. Eighty percent of isolated rim calcifications that are identified in renal masses with radiography are benign cysts, and 20% are malignancies. At the opposite end of the spectrum, renal masses that contain central, irregular calcifications (see Fig. 3-3 ) are likely to be malignant. Eighty-seven percent of these lesions are RCCs, and the remaining 13% are cysts complicated by previous infection or hemorrhage. Some renal masses contain both thin, peripheral calcifications and focal, central calcifications. Half of these renal masses are RCCs and the other half are simple cysts. Approximately 15% of RCCs contain calcifications that are visible on abdominal radiographs. Considering all of these variables, 60% of renal masses that contain calcifications that are visible on an abdominal radiograph, regardless of the calcification pattern, are RCCs. Besides complex renal cysts, other renal masses that frequently contain calcifications are focal xanthogranulomatous pyelonephritis (XGP), chronic perirenal hematomas, hemangiomas, aneurysms, and vascular malformations. Cross-sectional imaging helps to distinguish between benign and malignant calcified renal masses in most patients.
Box 3-5
Renal Mass Calcifications
Imaging Statistics
-
Up to 31% of RCCs contain calcium at CT
-
1%-2% of cysts contain calcium
CT, Computed tomography; RCC, renal cell carcinoma.
Other plain-film findings that may be important are skeletal abnormalities. RCC often spreads hematogenously to the skeleton, causing lytic skeletal lesions. These lesions sometimes grow slowly and lead to bubbly lesions that focally expand the bone. They can mimic other types of bone lesions, including metastases, primary bone neoplasms, and myeloma. Multiple osteomas, or bone islands, are another interesting type of skeletal abnormality sometimes seen in association with renal masses. Patients with tuberous sclerosis (TS) can have multiple osteomas, which are particularly predominant in the skull and spine. AMLs of the kidney develop in 80% of these patients and multiple renal cysts develop in a smaller percentage. Patients with TS do have an increased risk of developing RCCs.
Intravenous Urography
Renal masses were once often detected with intravenous urography (IVU), but this technique is rarely used today. The currently preferred techniques for renal mass detection are CT, MRI, and ultrasonography (US). However, if a mass is detected by urography, then some imaging features may be helpful to guide further evaluation. Expansile renal masses (see Box 3-3 ) lead to a focal bulge extending from the kidney, displacing normal renal structures ( Figs. 3-5 and 3-6 ). Contour abnormalities are optimally detected with nephrotomography performed during the nephrogram phase of the IVU. Large masses lead to calyceal splaying, stretching, and draping, whereas infiltrating renal lesions usually produce little, if any, parenchymal mass effect. However, within the infiltrated parenchyma, function is absent or greatly diminished, and therefore opacification in the involved region is diminished during the nephrogram phase. In addition, infundibular stricture with resulting hydrocalyx and calyceal amputation ( Fig. 3-7 ) are typical urographic findings associated with infiltrating renal masses. Because many of these masses arise or invade the calyces, calyceal filling defects, also known as an oncocalyx , may be evident on the intravenous urogram.
As mentioned earlier, excretory urography lacks sufficient specificity for accurately characterizing any renal masses as benign or malignant. Therefore every renal mass detected with or suggested by excretory urography must be imaged with another technique. The most cost-effective approach is to go directly to renal sonography. With this technique, 80% of detected renal masses are characterized as simple cysts, thus ending their diagnostic evaluation. The remaining 20% of renal masses require further study with CT or MRI.
Ultrasound
US is very useful in the detection of expansile renal masses, and in characterizing renal masses as solid or cystic. US, unfortunately, is of little value in detecting infiltrating renal lesions because these lesions may cause only subtle sonographic abnormalities or may not cause any abnormalities at all. Extensive infiltrating lesions often lead to secondary abnormalities, including hydronephrosis and vascular encasement with diminished flow to the area of involvement. Alteration of the normal central renal sinus echo complex may also be seen. These findings may be detectable with US and should suggest the presence of an infiltrating process.
Cross-Sectional Imaging
Cross-sectional imaging (i.e., CT, MRI) is the most useful imaging modality for the detection and characterization of renal masses. Expansile renal masses are routinely found with CT scanning. In addition, CT with contrast material infusion or MRI is extremely accurate in renal mass detection, characterization, and staging. Expansile renal masses 5 mm or larger are almost always detectable with these two modalities. Some renal masses can be undetectable on standard contrast-infused helical CT when scanning is performed in only one phase of contrast enhancement. When the kidneys are imaged during the portal venous phase of liver enhancement, renal enhancement is usually in the corticomedullary phase and may be insufficient for detection. In this phase, hypervascular cortical masses and hypovascular medullary tumors may be inconspicuous and undetectable. The optimal phase for renal mass detection with helical CT is the tubular or nephrographic phase. This typically occurs approximately 80 to 120 seconds after the initiation of intravenous contrast material injection. Virtually all renal tumors will be detectable during this phase of enhancement. CT angiograms can also be obtained with helical CT, obviating the need for later catheter angiography for surgical planning. Contrast material infusion with CT is essential for the detection of infiltrating lesions because they cause little or no contour abnormality (see Fig. 3-2 ). While detection of infiltrating lesions can be somewhat more difficult than detection of expansile masses, with contrast infusion, these lesions are usually detectable with CT. With MRI, intravenous contrast usually helps to detect and characterize many renal tumors. Unfortunately, neither CT nor MRI is completely reliable for characterization of infiltrating renal lesions, once detected because of the considerable amount of overlap of the CT and MRI findings of the various infiltrating lesions.
Angiography
Renal angiography, once a basic element in the diagnosis of renal masses, is of little value in the evaluation of most renal masses. Angiography has traditionally been reserved for mapping vascular supply to the kidney harboring a renal mass when a partial nephrectomy is contemplated. Because noninvasive techniques such as CT or MR angiography can be used to derive similar information on renal vasculature, these techniques have largely replaced catheter angiography for this indication.
Renal arteriography in combination with embolization may be useful in the treatment of some renal masses. Devascularization of a tumor may be performed before excision or ablation to reduce intraoperative blood loss or to enhance ablation efficacy, or to diminish symptoms from an inoperable renal malignancy. In rare cases, angiography may be useful in distinguishing among various renal masses. In particular, angiography may be an alternative to open biopsy in the diagnosis of infiltrating renal neoplasms. Urothelial neoplasms, inflammatory lesions, and infarcts are nearly always hypovascular or avascular. An uncommon subtype of RCC is the infiltrating variety. The fact that this tumor is usually very vascular distinguishes it from other infiltrating lesions. Therefore an infiltrating renal mass that is hypervascular strongly suggests an infiltrating RCC. This finding is important because RCC is traditionally treated with nephrectomy, whereas TCC is treated with nephroureterectomy, and many other infiltrating lesions are treated medically.
Classification of Renal Masses
For ease of understanding, renal masses can be separated into categories based on their growth pattern. The categories are solitary expansile masses, multiple expansile masses, and geographic infiltrating lesions.
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