Lymphoma of the kidneys is diagnosed at the time of autopsy in almost one half of cases. It is rarely suspected on conventional urologic studies, such as intravenous urography. However, renal involvement with lymphoma is commonly discovered in patients with known lymphoma, who may be diagnosed, staged, or followed by cross-sectional imaging, such as computed tomography (CT) scanning (see the images below) or magnetic resonance imaging (MRI). 
Patients with renal lymphoma may not be symptomatic; however, renal function laboratory studies occasionally reveal elevated blood urea nitrogen and creatinine levels, which may suggest an obstruction. Therefore, the patient may need to undergo radiologic evaluation. [2, 3, 4, 5, 6, 7]
Numerous diagnostic studies are available for the evaluation of renal involvement from lymphoma, including intravenous urography, ultrasonography, nuclear medicine, MRI, and CT scanning. [8, 9, 10] Although each technique may provide useful information in the detection, characterization, and staging of the disease, CT scanning remains the most sensitive, efficient, and comprehensive modality. [11, 12]
Urography provides information regarding involvement of the collecting system, the functional status of the kidneys, and the severity of hydronephrosis. Ultrasonography is a good screening examination, particularly for the assessment and follow-up of patients with hydronephrosis. In comparison, nuclear medicine studies, particularly those that use gallium-67 citrate, have a higher degree of specificity in the detection of lymphomatous infiltration of the kidneys. MRI provides information for characterizing the lesion, and it is also helpful in assessing the renal vascular pedicle.
CT scanning is the diagnostic modality of choice in patients with suspected renal masses (see the images below). CT scans depict renal involvement in most patients with lymphoma, often help in defining the extent of disease within and outside of the kidneys, and provide information for staging of the patient's underlying malignancy. In addition, the result of treatment can be assessed on follow-up CT scans. The CT scan examination must be done with intravenous contrast media. Therefore, if contrast media cannot be used in patients for reasons such as renal failure or allergy to iodine, MRI then becomes the diagnostic modality of choice.
Limitations of techniques
Intravenous urography can depict only the renal collecting system and suggestive large masses that have caused calyceal distortion. Small parenchymal masses and tumors around the kidney and in the retroperitoneum are often not demonstrated with this diagnostic examination.
On sonograms, the parenchymal masses are often hypoechoic, reflecting tissue homogeneity, and they can be misidentified as renal cysts.
Regarding nuclear medicine studies, although gallium-67 citrate is an isotope that often accumulates in lymphomatous tissue, it can also be taken up by inflammatory masses.
Nonenhanced CT scan studies or MRIs can cause masses to be missed, particularly when such masses are small.
Plain radiographs are limited when compared with other available modalities, such as CT scanning, ultrasonography, and MRI. Intravenous urography may show distortion of the collecting system as a result of a single or multiple renal masses. This finding cannot be differentiated from findings in simple cysts or renal cystic disease. However, intravenous urograms of renal lymphoma can demonstrate normal or near-normal findings (see the images below). [13, 14]
With urography, the degree of confidence for the diagnosis of renal lymphoma is low. If renal involvement is suggested on an intravenous urogram, contrast-enhanced CT scanning or MRI must be performed. If the urographic findings are considered normal but the clinical findings (eg, abnormal renal function) suggest renal involvement, a cross-sectional study is indicated.
In lymphomatous renal involvement, findings of an intravenous urogram can be normal and, therefore, falsely negative. With urography, a high rate of false-negative diagnoses is possible.
CT scans demonstrate a wide variety of appearances in renal lymphoma; the study patterns depend on the technique of CT scanning. Multidetector and spiral/helical CT scanning are universally accepted as the techniques of choice in patients with renal involvement, and they have a high degree of sensitivity in the detection of renal lymphoma. [15, 16, 17, 18, 19, 20, 21]
Breath-hold images can be quickly obtained during the multiple phases of renal enhancement after the intravenous injection of contrast medium. The nephrographic phase is the most important phase for renal parenchymal assessment. These images are essentially free of motion artifact and respiratory misregistration. This approach helps in accurately detecting and characterizing most renal masses, including smaller lesions that may involve the renal parenchyma in lymphoma.
In addition, the vascular phase of contrast enhancement enables characterization of the renal vasculature, which may be involved in renal cell carcinoma but uncommonly in renal lymphoma. Without intravenously administered contrast material, infiltrative forms of renal lymphoma can be entirely missed on CT scan studies. Also, CT scanning should not be limited to the cortical medullary phase of contrast enhancement because the lesions can be missed in this phase.
Typical imaging findings of renal lymphoma include multiple poorly enhancing masses, retroperitoneal tumors that directly invade the kidneys, a solitary mass, bilateral renal enlargement, and perirenal soft-tissue masses. Unless the renal lesions manifest in the setting of widespread lymphoma, percutaneous biopsy is indicated to differentiate lymphoma from metastases. Radiologists should be familiar with both the typical and atypical manifestations of renal lymphoma and should recommend imaging-guided percutaneous biopsy for diagnostic confirmation to avoid unnecessary nephrectomy. 
Multiple renal masses are most common in patients with lymphoma. The masses are typically bilateral, but they may also be unilateral. Multiple masses are seen in approximately 60% of patients, and they reflect advanced disease in patients who present for staging evaluation. Depending on the size and expansile nature of individual neoplastic deposits, little mass effect on the renal contour may be seen. Therefore, although renal involvement may be extensive, imaging findings can be subtle, and intravenous contrast enhancement is essential.
The masses range in size from a few millimeters to several centimeters and are seen as areas of low attenuation. Although the masses are often homogeneous on CT scans, they can be heterogeneous or low density, as well as mimic a complicated cyst. This is typically a sign of tumor necrosis inpatientsundergoingchemotherapy.In approximately 50% of patients, associated retroperitonealadenopathyis noted. (See the images below.)
Solitary masses are reported in approximately 10-20% of patients (see the images below). The mass can be as large as 15 cm and can cause significant distortion of the renal contour. Solitary lesions resemble other renal neoplasms that grow primarily by expansion, including renal cell carcinoma or isolated metastasis.
Continuous retroperitoneal extension into the kidney is a common pattern of renal involvement in lymphoma and is seen in approximately 30% of patients (see the image below). Continuous extension typically results in a large, bulky retroperitoneal mass that envelops the renal vasculature and invades the renal hilus. In most patients, the renal vascular pedicle is patent and engulfed by the retroperitoneal mass. Continuous extension of retroperitoneal involvement in the renal collecting system can often cause obstruction, and patients commonly present with hydronephrosis.
Rarely, perirenal disease is seen in an isolated form without involvement of the renal parenchyma or retroperitoneal nodes (see the images below). Occasionally, this feature is seen without compression of the renal parenchyma or functional impairment; such a finding is rare and virtually pathognomonic for perirenal lymphoma. A variety of CT scan manifestations of perirenal lymphoma, including renal sinus infiltration, thickening of the Gerota fascia, and perirenal masses, have been described. The normal renal parenchyma cannot be differentiated from the perirenal abnormality if intravenous contrast enhancement is not used in CT scanning.
Infiltrative disease is associated with lymphomatous proliferation within the interstitium of the kidney, which manifests as nephromegaly, with preservation of renal contour (see the image below). Diffuse infiltration is often bilateral and is seen in approximately 20% of patients. The diagnosis is often subtle and depends on the finding of global renal enlargement. Usually, kidneys infiltrated by lymphoma have poor function, but the disease is often clinically silent.
Atypical manifestations of renal lymphomatous mass, such as spontaneous hemorrhage, necrosis, heterogeneous attenuation, cystic transformation, and calcifications, are occasionally depicted on CT scans. Often, these findings are the result of previous treatment for renal lymphoma. More commonly, the findings of renal lymphoma regress after chemotherapy, and the kidneys regain a normal CT scan appearance.
In patients who are immunocompromised, the prevalence of lymphoma is increased. Lymphoma typically occurs in patients with human immunodeficiency virus infection or in secondarily immunosuppressed recipients of organ transplants. Lymphoma is particularly common in patients who have undergone cyclosporine therapy.
Typically, lymphoma in patients who are immunocompromised is of small, noncleaved cell or immunoblastic origin and has a B-cell phenotype. This condition often has an extranodal origin, and the most common sites include the central nervous system, bone marrow, gastrointestinal tract, lungs, heart, and liver. Renal involvement also occurs in these patients, but it is uncommon.
Degree of confidence
In a patient with known lymphoma, low-attenuating perirenal masses or nodules almost always represent renal lymphoma. On contrast-enhanced CT scans, these lesions have attenuation values higher than those of simple fluid. Contrary to typical primary renal cell carcinoma, lymphomatous masses are often homogeneous and do not display significant enhancement. If the renal mass is exophytic, it causes vascular displacement and not encasement; the latter feature is common in renal cell carcinoma.
Such findings may be associated with retroperitoneal adenopathy; however, when a single mass is found, their differentiation from renal cell carcinoma may become difficult, and occasionally, biopsy may become necessary. In perirenal lymphoma (halo sign on sonograms), the degree of confidence is higher.
Magnetic Resonance Imaging
MRI findings in lymphoma are similar to those seen on CT scans, and they are diagnostic to the same degree. Usually, lymphoma displays low signal intensity on T1-weighted MRIs, and it is either isointense or moderately hyperintense on T2-weighted images. Lymphomatous tissue may be minimally enhancing, but it does not enhance as much as normal renal parenchyma; therefore, the lymphomatous tissue remains hypointense relative to the kidney on contrast-enhanced T1-weighted MRIs (see the images below). [23, 24, 8, 21]
The degree of confidence of MRI for renal lymphoma depends on the technique that is used for scanning. On breath-hold high-resolution MRIs that are obtained with nonenhanced and contrast-enhanced sequences, MRI is probably equivalent to CT scanning.
When renal involvement is unilateral, it can mimic renal cell carcinoma (see image below). When diffuse renal infiltration is noted without a discrete lesion, the possibility of a false-negative finding increases.
Ultrasonography often demonstrates single or multiple renal masses that have low-level echoes (see the the first image below). Occasionally, the ultrasonographic findings may appear normal because of the small size of the renal nodules (see the second image below). In addition, varying degrees of hydronephrosis may be seen because of compression of the renal hilum that is caused by lymph nodes or obstruction of the ureters. [25, 26]
On sonograms, a perirenal hypoechoic halo is characteristic of perirenal lymphoma (see the image below).
The degree of confidence with the ultrasonographic assessment of renal involvement is lower than that of CT scanning or MRI; however, in patients with perirenal involvement, the pattern of a halo sign is a characteristic finding.
As a result of the homogeneous nature of large lesions, these lesions appear hypoechoic on ultrasonography, but without enhanced through-transmission. Occasionally, differentiating large lesions from a simple or a complicated cyst is difficult. If the lesions are small or if the nature of involvement is diffuse, the ultrasonographic findings can be normal.
Gallium-67 citrate has been used for diagnosing and staging lymphoma. This radioisotope concentrates in lymphoma; gallium is taken up by lymphomatous tissue in the kidneys, and it is also concentrated in inflammatory masses of the kidney. Therefore, obtaining the clinical history of patients is important. [9, 10, 25]
18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning has very high but not absolute specificity for malignant lymphoma. It has been reported that increased FDG uptake may also be observed in patients without active disease; in most cases, however, nonpathologic FDG accumulation is properly identified. Less frequently, inconclusive scans are encountered; these cases are usually caused by inflammation, which subsequently resolves. [27, 28]
The use of another agent, technetium-99m (99Tc)-labeled antibody (LL2), has been advocated in imaging and staging lymphomas. 
Testing with gallium-67 citrate has an overall sensitivity of approximately 80% (see the image below). However, in patients with high-grade lymphoma, gallium scanning has a higher degree of sensitivity. LL2 scanning also has a sensitivity of 80%; however, in patients with low-grade lymphoma, LL2 imaging usually has a higher degree of sensitivity.
Inconclusive scans are encountered with both gallium-67 citrate and FDG-PET. These cases are usually caused by inflammation, which subsequently resolves.
Usually, lymphoma is a hypovascular neoplasm; therefore, angiography cannot help in the diagnosis or staging of lymphomatous tumors. Thus, with contrast-enhanced CT scanning, the lesions remain hypovascular compared with the renal parenchyma when the contrast material is delivered to the kidney, via either intravenous injection for CT scanning or direct arterial injection for angiography. Therefore, the degree of confidence with angiography is low, and the degree of false-negative findings is high.
The degree of confidence is low in the diagnosis of renal lymphoma with angiography. False-negative diagnoses occur at a high rate.