Retroperitoneal liposarcoma is the most common primary malignant tumor arising from the retroperitoneal space, constituting approximately 45% of retroperitoneal sarcomas. It typically becomes symptomatic after reaching large size (average 20-25 cm) because the retroperitoneal space allows expansion. Well-differentiated liposarcoma (WDLPS/atypical lipomatous tumor) is the most common subtype, containing macroscopic fat mixed with non-fat solid components. Dedifferentiated liposarcoma (DDLPS) presents as an aggressive solid mass with no or minimal fat content. The coexistence of fat and non-fat (solid, calcified, necrotic) components on CT is the diagnostic clue. Surgical resection is the primary treatment, but high local recurrence rates (40-60%) are characteristic.
Age Range
40-70
Peak Age
55
Gender
Equal
Prevalence
Uncommon
Retroperitoneal liposarcoma originates from mesenchymal cells in retroperitoneal adipose tissue. MDM2 and CDK4 gene amplification (chromosome 12q13-15) plays a key role in tumorigenesis and is the shared genetic feature of both well-differentiated and dedifferentiated subtypes. In well-differentiated liposarcoma, tumor cells resemble mature adipocytes but show nuclear atypia — therefore the macroscopic fat component is dominant, and fat density regions (-10 to -120 HU) are seen on CT. Thickened septae and solid nodules reflect the fibromatous stroma and cellular areas. The dedifferentiation process involves transformation of fat cells into non-lipogenic sarcoma cells — these areas contain no fat, appear as high-density solid mass, and acquire aggressive biological behavior. The large expansion capacity of the retroperitoneal space allows the tumor to grow silently — the tumor initially displaces adjacent organs (kidney, pancreas, colon) and invades them in late stages. Neovascularization is visualized as enhancement in the solid components.
Sharp transition from macroscopic fat density to non-fat solid component within a single retroperitoneal mass — pathognomonic for DDLPS. Demonstrates coexistence of two different differentiation degrees.
Large (>15 cm) heterogeneous mass in retroperitoneal space containing fat density regions (-10 to -120 HU). Fat proportion varies: WDLPS >50% fat, DDLPS <25% or no fat. Borders well-defined but no true capsule — pseudocapsule from compressed surrounding tissue.
Report Sentence
Large heterogeneous mass measuring approximately ...x... cm in retroperitoneal space, containing fat density regions and solid components, consistent with liposarcoma.
Thickened (>2 mm) enhancing septae and/or solid nodules within fat component. Septae may show irregular thickness. Solid nodules 1-5 cm at soft tissue density (30-60 HU non-contrast, 60-100 HU enhanced). Critical for differentiating from simple lipoma — lipoma septae <2 mm and non-enhancing.
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Thickened enhancing septae and solid nodules within the fat component, findings favoring well-differentiated liposarcoma.
Non-fat, heterogeneously enhancing solid component within the tumor — characteristic of DDLPS. Soft tissue density (30-70 HU), may contain necrotic-cystic areas. Sharp boundary at fat-to-solid transition strongly suggests DDLPS. Larger solid component correlates with higher grade.
Report Sentence
Non-fat heterogeneously enhancing solid component adjacent to fat component, consistent with dedifferentiated liposarcoma.
On T1, high-signal fat component and intermediate-to-low signal solid components are seen together. On fat suppression, fat is suppressed while solid component retains signal. Chemical shift provides additional information for microscopic fat assessment.
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On T1, areas isointense to fat and non-fat solid components are seen together within the tumor, consistent with liposarcoma.
Solid component shows heterogeneous T2 signal — cellular areas intermediate, myxoid areas high, fibrous areas low signal. Necrotic areas very high T2 signal. Fat component high on T2, suppressed on fat-suppressed T2.
Report Sentence
The solid component shows heterogeneous T2 signal, reflecting cellular, myxoid, and fibrous areas.
The retroperitoneal tumor displaces adjacent organs (kidney, pancreas, colon, IVC, aorta). Kidney most commonly displaced. Vessel encasement indicates local invasion. Organ displacement without overt invasion suggests retroperitoneal origin.
Report Sentence
The retroperitoneal mass displaces the left/right kidney anterolaterally with preserved/lost fat plane between tumor and organ.
On FDG PET-CT, fat component shows low activity (SUVmax <2-3) while solid/dedifferentiated component shows increased FDG uptake (SUVmax 5-15+). Gradient at fat-to-solid transition. Valuable for detecting dedifferentiation and grading.
Report Sentence
Low metabolic activity in fat component, increased FDG uptake (SUVmax: ...) in solid component, consistent with dedifferentiated liposarcoma.
Criteria
Most common subtype (40-50%). Dominant macroscopic fat with thickened septae and solid nodules. Low grade, high local recurrence but rare distant metastasis. 5-year survival >80%.
Distinct Features
CT shows >50% fat, thickened septae (>2 mm), low FDG uptake (SUVmax <3). MRI shows dominant fat with T1 high signal.
Criteria
High-grade non-lipogenic sarcoma within WDLPS background. Fat-to-solid transition characteristic. Distant metastasis capacity (15-20%). 5-year survival 40-60%.
Distinct Features
CT shows fat-to-solid transition, heterogeneous enhancement, calcification (30%). PET SUVmax >5.
Criteria
Rare in retroperitoneum. t(12;16) FUS-DDIT3 translocation. Dominant myxoid stroma — near water density on CT, very high T2 on MRI.
Distinct Features
CT: homogeneous mass near water density. MRI: very high T2 signal.
Criteria
Rarest and most aggressive (5%). Minimal or no fat. Early metastasis. 5-year survival <30%.
Distinct Features
CT: aggressive solid mass without fat, frequent necrosis. Very high FDG on PET. Diagnosis histopathological.
Distinguishing Feature
Simple lipoma shows homogeneous fat density, septae <2 mm, no solid nodules. Liposarcoma has thickened septae and solid nodules.
Distinguishing Feature
Renal AML is intrarenal, usually <4 cm. Retroperitoneal liposarcoma is extrarenal, large, displaces kidney.
Distinguishing Feature
Adrenal myelolipoma is adrenal origin, fat+hematopoietic tissue, usually <10 cm. Liposarcoma solid component enhances differently and is larger.
Distinguishing Feature
Retroperitoneal lymphoma: homogeneous soft tissue density, no fat, sandwich sign. Fat component in liposarcoma is distinguishing.
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
3-monthPrimary treatment is wide surgical resection. Adjacent organ resection frequently required. Neoadjuvant radiotherapy may be considered. Recurrence 40-60% — 3-monthly CT/MRI follow-up recommended. Adjuvant chemotherapy may be considered for DDLPS. MDM2/CDK4 inhibitors under investigation.
Retroperitoneal liposarcoma requires wide surgical excision. Local recurrence is common. Dedifferentiated subtype has poor prognosis. Sampling of entire mass is important to determine percentage of dedifferentiated components.