Leukemic involvement of the kidney is rare and is seen more frequently in acute lymphoblastic forms (Pickhardt et al. 2000; Surabhi et al. 2008). The real frequency of renal involvement is difficult to assess, because of the limited contribution of imaging in staging (Pickhardt et al. 2000). Renal involvement can present as bilateral diffuse renal infiltration; more rarely, it presents as a focal expansile mass or a perirenal mass. Perirenal involvement is the result of extension of a renal mass in the perirenal space or more rarely as an isolated involvement (Pickhardt et al. 2000; Surabhi et al. 2008).

As imaging is nonspecific, clinical history may suggest the diagnosis, but percutaneous biopsy is required to achieve the diagnosis (Surabhi et al. 2008).

Granulocytic sarcomas (chloromas) are rare malignant tumors of granulocytic precursors that can present in up to 10 % of patients with acute myelogenous leukemia and only rarely in acute lymphocytic leukemia. On imaging studies, they present as focal hypovascular soft-tissue masses in one or both kidneys (Surabhi et al. 2008).

Two subgroups are distinguished: solitary lesions (plasmacytoma) and multiple lesions (multiple myeloma or Kahler’s disease). Extramedullary plasmacytoma is infrequent but can involve the kidney.

On imaging, plasmacytoma presents as a solitary infiltrative or expansile mass without specific radiological features.

In isolated renal plasmacytoma, diagnosis is usually based on the occurrence of Bence Jones proteinuria. In case of the absence of tumor-producing immunoglobulins, the diagnosis is only obtained by histopathologic analysis (Pickhardt et al. 2000).

Castleman disease is an infrequent idiopathic lymphoproliferative disorder which sporadically involves the retroperitoneum (Enomoto et al. 2007). Renal and perirenal involvement are rare. Unicentric and multicentric Castleman disease are distinguished. Multicentric Castleman disease is seen in association with Kaposi sarcoma and is frequently observed in patients with AIDS. Multicentric Castleman disease tends to be more aggressive and has a poor prognosis.

Imaging findings vary widely depending on the clinical presentation and histological subtype. Hepatosplenomegaly, lymphadenopathies, and ascites tend to be present.

Unicentric Castleman disease displays as a well-defined, homogeneous, solitary intra-abdominal mass (Surabhi et al. 2008).

See chapter “Renal lymphoma and renal sarcoma.”

Renal leiomyomas are rare benign mesenchymal tumors, found in ± 5 % of autopsy (Katabathina et al. 2010; Surabhi et al. 2008). They mostly originate from the renal capsule (smooth muscle cells) but can also originate from the renal pelvis (muscularis) or cortex (vascular smooth muscle) (Katabathina et al. 2010; Surabhi et al. 2008). Renal leiomyomas are divided into two groups: the more frequent, incidentally found, and asymptomatic cortical leiomyomas smaller than 2 cm and the large renal leiomyomas that generally become symptomatic.

The large leiomyomas appear more often in white women older than 20 years of age. In thin patients, the tumor may be palpable and cause flank pain and rarely hematuria (Tessler et al. 1998).

At imaging leiomyoma presents as a well-circumscribed mass lesion. Unenhanced CT shows a slight hyperdense mass. The enhancement of the lesion on CT is homogeneous. Cystic or myxoid degeneration, calcification, and hemorrhage can be present in larger lesions (Katabathina et al. 2010; Surabhi et al. 2008; Tessler et al. 1998). MRI shows a T2 and T1 homogeneous hypointense mass and heterogeneous signal in the larger tumors (cystic degeneration, calcifications, or hemorrhage (Katabathina et al. 2010)).

Frequently, a distinct plane is distinguishable between the tumor and the kidney without parenchymal distortion, confirming the capsular origin of the tumor. Occasionally, the mass can be extremely exophytic or attached to the cortex by only a small stalk (Wagner et al. 1997). However, imaging findings are nonspecific: a peripheral, exophytic, well-demarcated, solid renal mass in a middle-aged woman should suggest the possibility of renal leiomyoma (Tessler et al. 1998).

The differential list includes leiomyosarcoma and RCC. Leiomyosarcomas tend to be larger and less encapsulated, but sometimes they mimic a leiomyoma. Biopsy may be an option for decision on further management (Roy et al. 1998).

A primary renal lipoma is a very rare benign renal neoplasm. It is most often located in the cortex (Katabathina et al. 2010). As in leiomyoma, small and large lesions are distinguished. When large, they may become palpable and cause flank pain and, rarely, hematuria. Renal lipoma contains more nonadipose components than subcutaneous lipomas. These nonadipose components are mesenchymal elements or areas of fat necrosis and present as nodular or globular regions on imaging.

On imaging, a renal lipoma presents as a well-circumscribed fatty mass. On CT, this results in homogeneous negative density values (macroscopic fat). On MRI, the mass is T1 hyperintense on in-phase images and hypointense on out-of-phase images, T2-weighted images, and fat-saturated images. In smaller lesions, chemical shift characteristics allow the confirmation of subtle areas of fat more accurately compared to CT.

The differential diagnosis is angiomyolipoma. In lipoma, other soft-tissue components, with the exception of feeding vessels, are less prominent than in angiomyolipoma.

Larger lipomas are more heterogeneous due to areas of fat necrosis. Thus, the differential diagnosis expands to liposarcoma. Biopsy may then be required (aimed at nodular and globular areas) (Chiang et al. 2006).

See chapter “Benign solid renal tumors.”

Renal lymphangioma is a rare benign renal neoplasm, occurring uni- or bilaterally. This lesion is probably a developmental anomaly of the perirenal lymphatic system, rather than a tumoral proliferation.

There is no sex predilection and the tumor mostly affects adults. Evolution of this lesion is variable, with in some cases the presence of spontaneous resolution (Katabathina et al. 2010).

On imaging, a lymphangioma presents as a uni- or multilocular cystic lesion, focal or diffuse. Contrast-enhanced imaging can sometimes show peripheral or septal enhancement. It is located most often in the perirenal space or renal sinus (Katabathina et al. 2010; Surabhi et al. 2008). The lesion is usually T2 hyperintense and T1 hypointense at MR imaging. MRI signal intensity can vary if hemorrhage or debris is present in the lesion (Katabathina et al. 2010).

The perirenal space can be affected, and in the case of perirenal lymphangiomatosis, there are multiple cystic masses present bilaterally in the perirenal mesenchyme (Surabhi et al. 2008). Sometimes in association with a renal lymphangioma, linear fluid-filled structures are noted in the retroperitoneum, which are probably dilated lymphatic channels (Katabathina et al. 2010).

Juxtaglomerular cell tumors are very rare benign renin-producing tumors with a female predominance. They occur in adolescence or early adulthood (Beaudoin et al. 2008).

They originate from the myoendocrine cell of the juxtaglomerular apparatus (Katabathina et al. 2010).

They are an extremely rare cause of secondary hypertension.

Symptoms are secondary to the renin production and include moderate to severe headache (hypertension), polydipsia, polyuria, enuresis, and sometimes neuromuscular pains (hypokalemia).

On clinical examination, patients show moderate to severe hypertension. Elevated peripheral renin level with signs of secondary hyperaldosteronism is seen on blood analysis (Dunnick et al. 1983).

Tumorectomy or partial nephrectomy is the gold standard therapy with normalization of blood pressure soon postoperatively.

Histologically, reninoma has benign characteristics, but recent case reports have shown that vascular invasion and metastases may occur. Malignant behavior is observed in older patients with a large reninoma that has been present for many years (Beaudoin et al. 2008).

A reninoma most often presents as a small (2–3 cm) solitary mass on imaging studies, located subcapsularly. Occasionally, they can occur near the renal pelvis or in the perinephric space. The latter presumably occurs from perirenal embryonic rests (Dunnick et al. 1983).

They present rarely as a large mass which usually suggests the long-standing presence of this tumor.

Hemorrhage and necrosis can be present.

On ultrasound, a reninoma is homogeneously hyperechogenic; sometimes hypoechogenic areas are present due to hemorrhage or necrosis. On unenhanced CT scan, a reninoma is hypo- to isodense to the renal parenchyma and hypovascular in the arterial phase, despite profuse vascularity. This can be explained by a vasoconstriction induced by renin (Katabathina et al. 2010; Pedrosa et al. 2008).

On MRI, a reninoma is T1 hypointense, T2 hypointense, and hypovascular after contrast administration (Pedrosa et al. 2008).

Imaging features are very nonspecific. Therefore, the notice of hypertension, hyperreninism in the absence of renal artery stenosis, and the presence of a renal mass should suggest a juxtaglomerular cell tumor.

Other differential diagnoses for hypertension and hyperreninism and secondary hyperaldosteronism are renal artery stenosis, incipient malignant hypertension, other renin-secreting tumors (renin-secreting Wilms tumor or RCC, very rare renin-secreting lung, pancreatic, ovarian, liver, or orbital tumor), and other masses that compress the renal artery or parenchyma (larger than reninoma) (Dunnick et al. 1983; Garel et al. 1993).

Renomedullary interstitial cell tumor originates from the interstitial cells of the medulla. The lesions smaller than 5 mm are asymptomatic. Rarely, the lesions are large and then they may cause flank pain and hematuria.

Resection is advised when the lesion becomes symptomatic.

CT usually shows a small, hypodense, nonenhancing lesion, located in the renal medulla. If larger, the lesion may become polypoid and can protrude in the renal pelvis.

The lesion is T1 and T2 hypointense on MR imaging, due to the presence of collagenous tissue (Katabathina et al. 2010).

A solitary fibrous tumor is a spindle cell tumor usually originating in the pleura. Extrapleural locations including the kidney are rarely reported. It is suggested to be a tumor of primitive mesenchymal cells with features of multidirectional differentiation (Hanau and Miettinen 1995; Katabathina et al. 2010).

Renal solitary fibrous tumor usually arises from the renal capsule, but occasionally appears in the cortex, pelvis, or peripelvic connective tissue (Gelb et al. 1996; Katabathina et al. 2010). It is considered a benign tumor, but in rare occasions, malignancy can be present (Gelb et al. 1996). Resection is the treatment of choice (Katabathina et al. 2010).

On imaging, they present as a well-defined, large, homogeneous, and relatively hypovascular mass (Fig. 4). A typical cortical claw sign is absent, witnessing its extraparenchymal origin. Intralesional necrosis, calcifications, and cysts are infrequently encountered (Pedrosa et al. 2008; Katabathina et al. 2010). On MRI, the lesion is predominantly T2 hypointense and homogeneous T1 isointense to the renal capsule (Johnson et al. 2005; Pedrosa et al. 2008). Contrast enhancement is limited in the arterial phase and heterogeneous in the delayed phase (Johnson et al. 2005; Pedrosa et al. 2008).

Myelolipoma is a rare benign monoclonal tumor containing a mixture of normal hematopoietic cells and mature lipomatous tissue (Kumar and Duerinckx 2004). Normally, this tumor is located in the adrenal gland. Extra-adrenal myelolipomas are rare and occur mostly in perirenal or presacral area.

Age at presentation is the sixth decade in the extra-adrenal form, while adrenal myelolipoma is most often found (incidentally) in the fifth decade. Extra-adrenal myelolipomas show a female predominance.

On imaging, an extra-adrenal myelolipoma presents as a heterogeneous mass consisting of hypervascular soft-tissue and macroscopic fat. The aspect of the lesion depends on the dominating tissue component. At MR, high signal on T1- and T2-weighted images is present due to the fat component. Other areas show intermediate T1 and T2 signal due to marrow component (mimicking the signal of the spleen) (Heller et al. 2012).

The differential diagnosis with other fat-containing lesions is very difficult. Biopsy may be required (Surabhi et al. 2008).