Benign, Malignant, and Cystic Focal Renal Lesions

Ultrasonography.

The sonographic criteria for a simple cyst are well established and include (1) anechoic contents, (2) sharp smooth walls, (3) posterior acoustic enhancement, and (4) lack of internal blood flow.

Computed Tomography.

Findings characteristic of a simple renal cyst on CT are (1) water density (−20 to 20 Hounsfield units [HU]) on precontrast images, (2) smooth thin borders with a sharp interface with the adjacent renal parenchyma, and (3) does not enhance with contrast.

Magnetic Resonance Imaging.

A lesion can be categorized as a simple renal cyst on MRI, if the lesion (1) has a signal intensity that follows simple fluid, (2) has smooth thin borders with a sharp interface with the adjacent renal parenchyma, and (3) does not enhance on postcontrast images.

Clinical Implications.

A renal cyst that meets each of the criteria for any given imaging modality can be confidently diagnosed as a benign simple cyst and requires no additional evaluation ( Figure 63-1 ).

Radiography.

Intravenous urography and plain radiography are generally not sensitive techniques for identifying these tumors.

Computed Tomography.

The value of CT is in identifying any fat content present in these lesions because the presence of intratumoral fat is nearly pathognomonic of angiomyolipoma ( Figure 63-2 ). Lesions with a component demonstrating an attenuation value of less than 20 Hounsfield units (HU) are typical of angiomyolipoma on unenhanced CT. Typically, the lesion is a well-defined, cortical, heterogeneous mass with a fat component. The presence of hemorrhage may cause the lesion to appear poorly defined. Soft tissue attenuation of the lesion may be due to hemorrhage, smooth muscle, or fibrosis.

Approximately 5% of angiomyolipomas have insufficient fat to be recognized on CT and cannot be distinguished from renal cell carcinoma (RCC). Angiomyolipomas without visible fat appear homogeneous and demonstrate higher attenuation than adjacent normal renal parenchyma ( Figure 63-3 ). These lesions also can demonstrate contrast enhancement because of the relatively larger smooth muscle and vascular components.

Calcification is rarely seen but may be present after hemorrhage. In the presence of significant calcification, the diagnosis of angiomyolipoma should be reconsidered.

CT is useful to evaluate for retroperitoneal hemorrhage that may be associated with angiomyolipomas ( Figure 63-4 ). Blood products may obscure the fat content of the lesion, and in these cases carcinoma cannot be excluded.

Magnetic Resonance Imaging.

MRI can be used to depict intratumoral fat. Variable areas of high signal are seen on T1- and T2-weighted images owing to the fat content. However, high signal intensity on T1-weighted images also can be seen with high protein content or hemorrhage. In these cases, fat suppression techniques are useful. Israel and coworkers showed that angiomyolipomas also can be diagnosed using opposed-phase chemical shift MRI in which an India ink artifact at the mass/kidney interface or within a renal mass is suggestive of the tumor ( Figure 63-5 ). Clear cell RCC also may demonstrate signal loss on opposed-phase imaging; therefore, this is not a specific finding for angiomyolipoma.

Ultrasonography.

The typical ultrasound appearance of an angiomyolipoma is a well-defined hyperechoic mass without acoustic shadowing ( Figure 63-6 ). This echogenicity is not necessarily due to the fat component because some lesions that contain little or no fat also can be echogenic. Although RCC can appear echogenic, angiomyolipomas are more likely to show posterior shadowing. These differences, however, cannot distinguish between angiomyolipomas and carcinoma with the same degree of confidence as CT.

Nuclear Medicine.

Nuclear medicine has a limited role in the assessment of angiomyolipoma. When surgical management is indicated, it can be used to assess differential renal function.

Angiography.

Angiography characteristically demonstrates clusters of saccular microaneurysms or macroaneurysms. Other findings on angiography include hypervascularity, venous pooling with a whorled appearance, and lack of arteriovenous shunting ( Figure 63-7 ).

Imaging Algorithm.

Thin-section noncontrast CT is the preferred and most widely available method for detection of angiomyolipoma because demonstration of fat is pathognomonic. Although MRI can detect fat within an angiomyolipoma, it is not as sensitive as CT for detecting fat in small lesions. Opposed-phase chemical shift MRI has been investigated and found to be useful as a method for detecting angiomyolipomas ( Table 63-1 ).

Radiography.

Plain radiographic findings are nonspecific and include a soft tissue mass distorting the renal silhouette with displacement of the fat planes. Calcification is an uncommon feature.

Angiography.

The classic findings are a spokewheel arrangement of vessels, homogeneous dense tumor blush during the capillary phase, and sharp demarcation from the kidney. Bizarre neoplastic vessels are absent, in contrast to RCC.

Computed Tomography.

An oncocytoma typically appears as a well-defined solid renal mass with smooth margins. A central stellate scar is seen in one third of cases, but this is a nonspecific finding that cannot be differentiated from central necrosis in RCC.

On noncontrast CT, oncocytoma is isodense or hyperdense relative to normal renal parenchyma. On the nephrographic phase of contrast-enhanced CT, this tumor is most commonly hypodense to renal parenchyma ( Figure 63-8 ).

Magnetic Resonance Imaging.

Typically, oncocytoma demonstrates low signal on T1-weighted images and high signal on T2-weighted images. The central scar, if present, shows as a low T1 and a low T2 signal, in contrast to necrosis in RCC, which usually has a low T1 and a high T2 signal.

After administration of a contrast agent, oncocytoma typically demonstrates homogeneous enhancement. The central scar generally does not enhance ( Figure 63-9 ).

Ultrasonography.

On ultrasonography, an oncocytoma shows as a well-defined hypoechoic to isoechoic mass ( Figure 63-10 ). If visualized, the central scar may appear echogenic. Doppler imaging may show central radiating vessels in a spokewheel distribution.

Nuclear Medicine.

Nuclear medicine is not routinely used in the evaluation of renal tumors.

Positron Emission Tomography With Computed Tomography.

PET/CT is not routinely performed for evaluation of oncocytoma. It may show equal to slightly higher uptake of fluorodeoxyglucose (FDG) than normal renal parenchyma.

Imaging Algorithm.

Contrast-enhanced CT is the examination of choice for evaluating solid renal masses. Evaluation for vascular involvement, lymphadenopathy, and extent of the lesion is possible ( Table 63-2 ).

Radiography.

A large mass may be seen involving the entire kidney. Calcification is uncommon, and radiography is not a reliable modality for evaluating for the presence of renal tumors.

Computed Tomography.

A solid, homogeneous mass arising from the kidney that may replace all or part of the involved kidney. There can be areas of necrosis, although this is uncommon. The tumor does not enhance after contrast agent administration. However, entrapment of nephrons within the mass may cause excretion of contrast agent within the mass. No calcification is evident.

Magnetic Resonance Imaging.

There is low signal intensity on T1-weighted imaging, and the tumor is typically nonenhancing.

Ultrasonography.

A well-circumscribed, homogeneous and hyperechoic mass is evident. Concentric hypoechoic and hyperechoic rings are a helpful imaging feature.

Imaging Algorithm.

Prenatal or pediatric ultrasonography is usually the first imaging study performed when the abdominal mass is palpated. Ultrasonography is easily and widely available; it is inexpensive and involves no ionizing radiation. CT has an important role in evaluating recurrent or metastatic disease.