• Solitary soft tissue masses (RCC, lymphangioma,hemangioma)
• Rindlike soft tissue lesions (lymphoma, retroperitoneal fibrosis, Erdheim Chester disease)
• Multifocal soft tissue masses (metastases)
• Masses containing macroscopic fat (AML, extramedullary hematopoiesis)

Neoplastic and Nonneoplastic Proliferative Disorders of the Perirenal Space: Cross-sectional Imaging Findings
Surabhi VR et al
RadioGraphics 2008; 28:1005-1017

“Extramedullary haematopoiesis (EMH) is the production of blood elements outside of the normal location. EMH occurs when there is either insufficient production or poor quality of the blood elements produced. Insufficient production of blood elements occurs when there is replacement of the bone marrow, most commonly caused bymyelofibrosis (the most common cause of EMH overall), diffuse metastatic disease, and leukemia. Examples of insufficient quality of blood products include haemoglobinopathies, most commonly sickle cell anaemia and thalassemia.”
Extramedullary haematopoiesis: radiological imaging features 
A.S. Roberts et al.
Clinical Radiology 71 (2016) 807-814

“Primary renal lymphoma without systemic extension is a rare clinical occurrence and it accounts for less than 1% of extranodal lymphomas. As the renal parenchyma does not normally contain lymphoid tissue, the development of primary renal lymphomas is thought to originate from lymphatic-rich renal capsule or perirenal fat with subsequent extension into the renal parenchyma. Secondary renal lymphomas may commonly occur in patients with advanced stage disease. Autopsy series of patients with lymphoma showed renal involvement in 30%–60% of the cases. Renal lymphomas are usually B cell non-Hodgkin type lymphomas. Secondary lymphomatous involvement of the kidneys is usually clinically silent, however, in a small percentage of patients flank pain, hematuria, and rarely renal failure may be observed.”
Computed tomography and magnetic resonance imaging of peripelvic and periureteric pathologies.
Gumeler E et al. 
Abdom Radiol (NY). 2018 Sep;43(9):2400-2411

“Renal involvement typically presents as solid solitary or multiple lesions within the renal parenchyma. Perirenal or diffuse parenchymal involvement may also be seen some patients. Isolated renal sinus involvement is rare and peripelvic involvement occurs secondary to tumoral extension from the conglomerated retroperitoneal lymph nodes. Ureteral obstruction may be frequently observed in patients with periureteral extension. Renal vascular occlusion or thrombosis is rare despite extensive encasement of these structures, owing to the soft and pliable nature of the lymphoma .”
Computed tomography and magnetic resonance imaging of peripelvic and periureteric pathologies.
Gumeler E et al. 
Abdom Radiol (NY). 2018 Sep;43(9):2400-2411

“CT demonstrates low attenuated lymphomatous infiltration along renal sinus and ureteral wall while this infiltration typically manifests as hypointense on both T1W and T2W images on MRI scans. This infiltrative growth pattern is reminiscent of transitional cell carcinomas (TCCs) and other inflammatory processes such as XGP. Based on these similarities, both TCC and XGP should be considered in the differential diagnosis. Mild and homogeneous post-contrast enhancement may be a helpful imaging hint to differentiate peripelvic and periureteral lymphoma from these disorders.”
Computed tomography and magnetic resonance imaging of peripelvic and periureteric pathologies.
Gumeler E et al. 
Abdom Radiol (NY). 2018 Sep;43(9):2400-2411

“The perirenal space is a retroperitoneal space that is limited anteriorly by the anterior renal fascia (Gerota fascia) and posteriorly by the posterior renal fascia (Zuckerkandl fascia).These two fasciae fuse to form the lateroconal fascia laterally and blend loosely with the periureteric connective tissue medially. Superiorly, the two fasciae are fixed to the diaphragmatic fascia above the adrenal glands; inferiorly, they blend with the iliac fascia.The anterior and posterior renal fasciae enclose a gradually tapering conelike space produced by the embryologic ascent of the kidneys from the pelvis to the adult retroperitoneal position.”
Neoplastic and Non- neoplastic Proliferative Disorders of the Peri-renal Space: Cross- sectional Imaging Findings 
Surabhi VR et al.
RadioGraphics 2008; 28:1005–1017