Clinical manifestations

Clinically, patients are relatively asymptomatic however can present with flank pain, hematuria, night sweats, and fever. Acute renal failure rarely manifests as the initial finding in cases of diffuse involvement. And while renal lymphoma can respond to treatment similar to lymphoma elsewhere in the body, recurrent renal lymphoma carries a poorer prognosis [9]. Furthermore, patients who present with acute renal failure in the setting of lymphocytic infiltration of the renal parenchyma can improve their renal function with the treatment of the underlying lymphoma, but typically not back to baseline [10].

Role of multimodality imaging

CT is the imaging modality of choice for the initial evaluation of patients with suspected lymphoma. The advantages of CT include high sensitivity for detection of renal lesions, extrarenal tumor extension, and involvement of other organs. Intravenous contrast is essential for the detection of subtle lesions; imaging in the late arterial phase is helpful in evaluating the vasculature and differentiating lymphoma from hypervascular primary renal tumors. The nephrographic phase is essential in detecting small lesions. Lymphomatous deposits tend to enhance less than the renal cortex and appear homogenous.

MRI is useful in demonstrating renal and perirenal disease, however, the role of MRI in evaluating renal lymphoma is less clearly defined in the literature. Renal lymphomatous tumors appear as T1 hypointense and T2 iso- or hypointense relative to the renal cortex. On post-contrast MRI, renal lymphoma enhances less than the renal parenchyma, with some lesions demonstrating progressive enhancement on delayed imaging [4].

Ultrasound is often the first imaging exam in patients presenting with renal insufficiency or flank pain. However, ultrasound is inferior to CT and MRI in detecting the presence of disease, number of lesions, and extrarenal disease. Lesions typically appear homogenous and hypoechoic. Color Doppler imaging may demonstrate little internal vascularity within the lesion, which tends to displace vessels rather than invade them [11]. The presence of vascular thrombus in the renal vein or inferior vena cava is atypical for lymphoma and alternative diagnoses should be considered. Contrast-enhanced ultrasound (CEUS) is useful in characterizing lesions and can differentiate between solid tumors, pseudo-lesions, and complex cysts [12]. However, differentiating renal lymphoma from other malignant renal masses is typically not feasible [13].

PET/CT is currently the gold standard for the staging of lymphoma and the detection of recurrent disease [14]. Its advantage in detecting the metabolic activity of tumors makes it more sensitive and specific than conventional anatomic imaging [15].

Multiple lesions

The most common imaging finding in renal lymphoma, occurring in 50–60% of cases, is multiple solid parenchymal masses [16]. These are more commonly bilateral although multiple unilateral lesions can also occur [17].

On unenhanced CT, lesions typically have higher attenuation than the surrounding renal parenchyma. Lesions tend to be homogeneous in appearance. (Fig. 1).

Fig. 1
figure 1

Multiple renal masses of renal lymphoma. Axial contrast CT (a) demonstrates small, hypoenhancing masses (arrows) in the right kidney. Axial fused PET/CT image (b) demonstrates F18-FDG avidity of these masses (arrows) consistent with lymphoma in this patient with known diffuse large b-cell lymphoma

The primary differential diagnosis for multiple renal masses includes metastatic disease, commonly from lung, breast, gastric cancer, and melanoma [18]. Multiple synchronous renal cell carcinomas (RCCs), particularly papillary and chromophobe subtypes, can mimic multifocal renal lymphoma. Image-guided biopsy may be needed to differentiate lymphoma from RCC, as the management for lymphoma consists of chemotherapy, not nephrectomy.

Benign differential diagnoses to consider in the setting of multiple renal masses include pyelonephritis, abscesses, renal infarcts, IGG-4 related renal disease, and extramedullary hematopoiesis. The presence of perirenal fascial thickening and infiltration of perinephric fat is nonspecific and has been observed in both inflammatory processes and lymphoma.

Solitary lesion

In approximately 10–25% of patients, renal lymphoma will present as a solitary mass [1, 4, 8]. When there is a lack of lymphomatous involvement elsewhere, a prospective diagnosis can be challenging (Fig. 2). Conversely, when retroperitoneal lymphadenopathy and splenic disease are present, these are helpful clues in diagnosing secondary renal lymphoma (Figs. 3 and 4).

Fig. 2
figure 2

Solitary mass of renal lymphoma. Axial contrast CT (a) demonstrates a solitary mass (arrow) in the right kidney with avid radiotracer uptake on F18-FDG PET/CT (b), as well as retroperitoneal adenopathy (arrowhead). This is consistent with solitary renal lymphomatous mass in this patient with marginal zone lymphoma. The marked FDG avidity differentiates it from typical renal cell carcinoma which can be mildly FDG-avid

Fig. 3
figure 3

Solitary mass of renal lymphoma. Coronal CT delayed post contrast image (a) demonstrates a hypoenhancing right lower pole renal mass with hypoechoic appearance on ultrasound (b) and marked F18-FDG avidity on PET/CT (c) (arrows). Additional FDG avid supradiaphragmatic and extensive infradiaphragmatic lymphadenopathy is helpful in confirming the diagnosis of lymphoma (arrowheads)

Fig. 4
figure 4

Solitary mass of renal lymphoma. Coronal CT post contrast image (a) demonstrates a solitary hypoenhancing left lower pole renal mass (arrow). Note the mass-like infiltration of the spleen (arrowhead) and retroperitoneal lymphadenopathy. PET/CT (b) shows the marked F18-FDG avid bilateral renal masses (arrows), retroperitoneal lymphadenopathy, and splenic mass (arrowheads), consistent with the diagnosis of large b-cell lymphoma

Other differential diagnoses for a solitary renal lesion include solitary metastasis and other benign etiologies such as focal pyelonephritis, abscess, or infarct.

Direct extension

Direct lymphomatous renal involvement from retroperitoneal lymphadenopathy is the second most commonly observed pattern of renal lymphoma, documented in approximately 25–30% of cases [5]. These patients usually have widespread disease with bulky tumors invading the perinephric space, displacing or invading the adjacent kidney. Resultant hydronephrosis from entrapment of ureters is common. However, vascular occlusion or thrombosis of renal arteries and veins is rare (Fig. 5).

Fig. 5
figure 5

Direct extension from retroperitoneal adenopathy of renal lymphoma. Grayscale ultrasound image (a) demonstrates mild hydronephrosis (arrowhead) of the left kidney. Spectral Doppler ultrasound (b) demonstrates a hypoechoic mass (white arrow) arising from the lateral left kidney with arterial waveform. Axial contrast CT (c) confirms the left renal mass (white arrow) with additional extensive retroperitoneal and peritoneal infiltrative disease encasing the left renal hilum. Note the maintained flow and normal caliber left renal artery (black arrow) despite tumor burden, characteristic of lymphoma. This patient is known to have follicular lymphoma with high grade B-cell lymphoma transformation

In the absence of a known diagnosis, other retroperitoneal malignant etiologies such as sarcomas may be considered. Other benign etiologies mimicking this process include IGG-4-related disease and retroperitoneal fibrosis (RPF) [19].

Diffuse renal infiltration or nephromegaly

Renal lymphoma resulting in nephromegaly without distortion of the normal reniform shape has been reported in 20% of cases. Lymphomatous proliferation in the interstitium of the kidney results in nephromegaly. This may be disseminated or limited to the kidneys and may be unilateral or bilateral (Figs. 6 and 7). Patients may present with acute renal failure from the destruction of the normal renal architecture.

Fig. 6
figure 6

Nephromegaly of renal lymphoma. Coronal noncontrast CT demonstrates infiltrative renal lymphoma with a diffusely enlarged left kidney replaced by tumor (arrowheads). Note the encasement and deformity of the pelvocalyceal system by tumor. This was pathologically confirmed as marginal zone lymphoma

Fig. 7
figure 7

Nephromegaly of renal lymphoma. Coronal noncontrast CT (a) demonstrates symmetric marked enlargement of the bilateral kidneys from lymphomatous infiltration in this patient with mantle cell lymphoma. Concurrent PET/CT in 9/2020 (b) demonstrates marked F18-FDG avid bilaterally enlarged kidneys as well as extensive supradiaphragmatic and infradiaphragmatic lymphadenopathy. Follow-up F18-FDG PET/CT in 11/2020 (c) following chemotherapy shows complete metabolic and radiologic response with normalized renal size and activity

On CT, the kidney appears enlarged, often with heterogeneous enhancement, loss of normal corticomedullary differentiation, and possible infiltration of the renal sinus fat [4].

Differential considerations include urothelial carcinoma, medullary renal cell carcinoma, acute autoimmune nephritis, and severe pyelonephritis [20].

Perinephric space disease

Isolated perinephric lymphoma is unusual and reported in less than 10% of cases [6, 10]. The imaging findings include limited thickening of Gerota’s fascia, plaques and nodules within the perirenal space, or a rind of perinephric soft tissue thickening with invasion or compression of the normal renal parenchyma (Fig. 8) [4].

Fig. 8
figure 8

Perinephric space infiltration of renal lymphoma. Axial CT of the abdomen (a) demonstrates an infiltrative homogenous mass (arrows) in the left retroperitoneum enveloping the left kidney with preservation of the left renal parenchymal anatomy. Additional thickening of Gerota’s fascia and Zuckerkandl fascia as well as retroperitoneal tumor infiltration along the pancreatic tail and splenic hilum are present (b). Note the preservation of normal caliber splenic vein and artery (arrowheads). Due to the unusual tumor morphology, the left renal mass was biopsied and consistent with diffuse large B-cell lymphoma

The differential diagnosis includes sarcoma, metastases to the perinephric space, and benign conditions such as pancreatitis, perinephric hematoma, RPF, amyloidosis, and extramedullary hematopoiesis (EMH).

Renal sinus involvement

Lymphoma can preferentially affect the renal sinus, though this is a very rare phenomenon and the exact incidence is not well documented.

On imaging, the renal sinus is replaced by a homogenous soft tissue mass, often resulting in mild hydronephrosis relative to the size of the tumor, due to its pliable nature. Vascular encasement or displacement is commonly seen (Fig. 9). On US, it may be difficult to differentiate the renal sinus tumor from heterogenous renal sinus fat due to its poorly defined margins [4, 21].

Fig. 9
figure 9

Renal sinus involvement of renal lymphoma. Coronal MRI post contrast (a) and T2 weighted (b) images demonstrate a T2 hypointense, hypoenhancing infiltrative left renal mass (arrows) invading the upper pole collecting system and lower kidney. Note the resultant mild hydronephrosis relative to tumor size. PET/CT (c) shows the marked F18-FDG avid left renal mass (arrow) as well as splenic masses (arrowhead). This was biopsy-proven high-grade B-cell lymphoma

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