Lesions in the retroperitoneum:

1. Sarcomas- heterogeneous group of malignant tumors of mesenchymal origin  that originate from the soft tissues rather than bone
   

   Fig 6: Axial arterial phases CT image of a 56-year-old man illustrating a large left retroperitoneal mass with heterogeneous enhancement that appears to arise from the left psoas muscle. It extends laterally, anteriorly displacing the bowel, which appears separate. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
   . Liposarcoma  is the most common type of retroperitoneal sarcoma, accounting for 40% of cases. It often grows to large sizes (up to 20 cm) before being detected, as symptoms typically arise from mass effect. The well-differentiated subtype is relatively easy to identify on imaging due to its high fat content. It commonly exhibits internal septations and nodularity with mild-to-moderate contrast enhancement (6)
   

   Fig 5: Well-differentiated liposarcoma. Axial venous CT images of a 80-year-old man reveals a mass adjacent to the right kidney, retroperitoneal, with an inhomogeneous structure showing fatty component and soft tissue with minimal contrast uptake at this level. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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2. Erdheim-Chester disease is a rare, non-Langerhans cell histiocytosis with non-familial, multisystem involvement and widely varying severity (4). ECD can be distinguished from primary RPF by the presence of "hairy kidneys" (bilateral, symmetrical dense infiltration of the perinephric space), a "coated aorta" (circumferential periaortic involvement), and the lack of pelvic ureter and IVC involvement (11)
   

   Fig 7: Erdheim-Chester disease (A), (B), (C) Axial venous phase CT, (D) delayed phase CECT illustrating irregular symmetric infiltration of the bilateral perirenal and pararenal spaces (hairy kidney appearance), coated renal vessels, IVC, abdominal aorta and involvment of both adrenals. Proximal ureteral infiltration with slightly delayed excretion on the right (shown on image (D)) . Patient had pericardial effusion (not shown). References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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3. Retroperitoneal fibrosis (RPF)  is a rare condition, more prevalent in males, and classified as either idiopathic or secondary. Imaging typically shows homogeneous soft tissue attenuation encasing the abdominal aorta and iliac arteries, with or without involvement of adjacent structures like the ureters and inferior vena cava. On nonenhanced CT, retroperitoneal fibrosis typically has attenuation values similar to the psoas muscle, and the fat plane between them may be obscured (3). After intravenous contrast administration, enhancement patterns vary by disease stage—early stages often show strong enhancement, whereas late or inactive stages may have minimal or no enhancement (5). Characteristic imaging findings include hydronephrosis and medial deviation of the mid-ureters, known as the "maiden waist sign," which appears as a smoothly tapering ureteral narrowing at the L4-L5 level (8)
   

   Fig 8: Retroperitoneal fibrosis (A), (B) Non-enhanced axial CT images of a male patient that shows homogeneous soft tissue attenuation encasing the abdominal aorta and iliac arteries, without involvement of adjacent structures . References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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4. Castleman’s disease is a non-neoplastic lymphoproliferative disorder that can affect lymph nodes or extranodal sites. Based on localization, it is categorized as either unicentric or multicentric. On contrast-enhanced CT, unicentric Castleman’s disease (UCD) typically presents as a well-defined, hypervascular soft tissue mass with homogeneous, intense enhancement. The lesion is usually non-invasive, and calcifications may be seen in some cases. It is most commonly located in the mediastinum (70%), followed by the abdomen, retroperitoneum, and pelvis (10-15%)
   

   Fig 9: Castleman’s disease . Axial venous phase of a male patient with multiple lumbo-aortic adenopathies with uknown primary origin shows a well-defined, non-invasive, soft tissue mass with homogeneous, moderate enhancement, without calcifications. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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5. Pancreatic acinar cell carcinoma (ACC) is a rare exocrine tumor, accounting for less than 1% of all pancreatic neoplasms. On contrast-enhanced CT, ACC typically presents as a well-defined, large, lobulated mass with mild to moderate enhancement . It rarely causes pancreatic ductal obstruction or significant desmoplastic reaction. Larger tumors may show areas of necrosis, hemorrhage, or cystic degeneration, contributing to a heterogeneous appearance. Calcifications are uncommon . ACC has a high tendency for early metastasis (14)
   

   Fig 10: Pancreatic acinar cell carcinoma. Axial venous phase CT scan of a male patient reveals a pancreatico-duodenal infiltrative mass tumor that invades the D3 and D4 portions of duodenum (B). The tumor incorporates the superior mesenteric artery (C, D). No dilatation of the Wirsung duct. The parenchyma in body and tail of the pancreas was normal. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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6. Perinephric myxoid pseudotumor of fat (PMPF) is a rare, benign lesion that can mimic malignant perinephric masses (9) . Approximately 82% of lesions contain macroscopic fat, appearing as areas of low attenuation. In about 18% of cases, particularly those involving renal transplants, macroscopic fat is absent, and the mass may appear as a soft-tissue density lesion (15)
   

   Fig 11: Perinephric myxoid pseudotumor of fat. Axial arterial phase CT scan of a female patient with left iliac fossa pain reveals a predominantly lipomatous mass located perirenally with areas of para-fluid densities adjacent to it. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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7. Retroperitoneal leiomyomas are rare benign smooth muscle tumors occurring in the retroperitoneal space . On contrast-enhanced CT, these tumors typically present as well-defined, homogeneous solid masses with mild to moderate enhancement (10). They may cause displacement of adjacent structures without evidence of invasion. Unlike their malignant counterparts, retroperitoneal leiomyosarcomas, leiomyomas generally lack significant cystic degeneration or necrosis (13). However, distinguishing between benign and malignant retroperitoneal smooth muscle tumors based solely on imaging can be challenging, and histopathological evaluation is often necessary for definitive diagnosis (7)
   

   Fig 12: Retroperitoneal leiomyoma. Axial arterial phase CT scan of a female patient shows a well-defined, homogeneous solid mass adjacent to the right adrenal gland. No calcifications were present. In the delayed phase (not shown), this tumors showed progressive enhancement. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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8. Urothelial carcinoma of the renal pelvis and calyces, commonly known as upper tract urothelial carcinoma (UTUC), is a malignancy arising from the urothelial lining of the renal collecting system. It accounts for approximately 5-10% of all urothelial cancers and presents unique challenges in diagnosis and management (16). Diffuse thickening of the renal pelvis or ureteral wall is indicative of UTUC. On excretory phase, it often presents as a filling defect within the renal pelvicalyceal system or ureter. In advanced cases, UTUC may present as an infiltrative mass, causing distortion of the renal collecting system, but the contour of the involved kidney is typically preserved, which helps differentiate UTUC from renal cell carcinoma . In nephrographic phase, the renal parenchyma enhances, and the collecting system is opacified. UTUC may appear as an enhancing soft-tissue mass within the collecting system. The nephrographic phase is particularly useful for detecting tumors that are enhancing and located within the renal parenchyma (17)
   

   Fig 13: Urothelial carcinoma of the renal pelvis and calyces. A), (B) Axial venous phase CT, (C) delayed phase CECT scan of a female patient illustrating a mild enhancing tissue mass centered at the level of the right renal pelvis with reduced contrast uptake compared to the renal parenchyma. In picture C it is shown a filling defect in the renal phase. Thickening of proximal ureteral wall was present. References: Department of Radiology, Spitalul Universitar de Urgenta Militar Central "Dr. Carol Davila," Bucharest, Romania
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