Undifferentiated pleomorphic sarcoma (UPS), formerly known as malignant fibrous histiocytoma (MFH), is a high-grade malignant mesenchymal tumor showing no specific cellular differentiation. It ranks third among retroperitoneal sarcomas after liposarcoma and leiomyosarcoma. Typically seen between ages 50-70 with slight male predominance. Histologically, it contains pleomorphic spindle and round cells, atypical mitoses, and storiform pattern; it has no specific immunohistochemical markers (diagnosis of exclusion). On imaging, it appears as a large, heterogeneous, aggressive mass; definitive distinction from liposarcoma/leiomyosarcoma usually requires biopsy. Prognosis is poor; 5-year survival ranges from 30-50%.
Age Range
40-75
Peak Age
60
Gender
Male predominant
Prevalence
Rare
UPS is a malignant tumor arising from mesenchymal stem cells but showing no differentiation toward a specific somatic cell lineage. Tumor cells demonstrate intense proliferation with TP53, RB1, and CDKN2A gene mutations/deletions playing roles in pathogenesis. Storiform pattern (spindle cells arranged in whorled fashion) is the characteristic histological finding. The heterogeneous imaging features reflect the coexistence of varying cellular density, necrosis, hemorrhage, and myxoid changes. Neovascularization is prominent — viable portions show intense enhancement. Necrosis can reach 30-50% of tumor volume resulting from rapid growth exceeding vascular capacity. Myxoid degeneration areas show markedly high signal on T2.
The diagnostic characteristic of UPS is the ABSENCE of specific diagnostic findings: no macroscopic fat (not liposarcoma), no IVC involvement (not leiomyosarcoma), no calcification (not chondrosarcoma), no neural foraminal widening (not neurogenic tumor). This sum of negative findings suggests UPS.
On contrast-enhanced CT, a large, irregularly marginated, heterogeneously enhancing retroperitoneal mass is seen. Unlike liposarcoma, it does NOT contain macroscopic fat density (areas between -30 and -120 HU). Solid components show moderate to intense enhancement. Necrotic areas are prominent and may comprise a large portion of the mass. Calcification is rare and minimal with low diagnostic value.
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A large heterogeneously enhancing mass without macroscopic fat is seen in the retroperitoneal space with prominent necrotic areas; high-grade sarcoma (UPS) should be considered.
On T2-weighted images, UPS shows markedly heterogeneous signal. Solid components show intermediate T2 signal, myxoid areas show very high T2 signal (near-fluid), necrotic areas show high T2 signal, and hemorrhagic foci show low T2 signal. Myxoid change areas are typically prominent showing fluid-like high signal on T2 — this is one of the most distinguishing MRI findings of UPS.
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The mass demonstrates markedly heterogeneous signal on T2-weighted images containing solid, myxoid, necrotic, and hemorrhagic components.
On T1-weighted images, UPS typically shows isointense to slightly hypointense signal relative to muscle. Hemorrhagic foci appear hyperintense on T1. Myxoid areas appear hypointense on T1 (high water content). Necrotic cavities show low T1 signal. Macroscopic fat signal is not expected — this is important in distinction from liposarcoma.
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The mass shows isointense signal to muscle on T1-weighted images with hemorrhagic foci and absence of fat signal.
On DWI, solid viable components of UPS show marked diffusion restriction (high DWI signal, low ADC). ADC values are typically in the range of 0.6-1.1 × 10⁻³ mm²/s — expected for high-grade sarcomas. Necrotic and myxoid areas do not show diffusion restriction. ADC value inversely correlates with tumor grade (low ADC → high grade).
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Marked diffusion restriction is seen in the solid components of the mass, consistent with high-grade sarcoma.
In the portal venous phase, peripheral viable tumor tissue of UPS shows heterogeneous enhancement while central necrotic areas remain non-enhancing. This peripheral enhancement-central necrosis pattern is typical for rapidly growing aggressive sarcomas. In the delayed phase, enhancement may show slow centripetal progression (fibrous stroma enhancement). Invasion or displacement of surrounding structures (psoas, kidney, bowel loops) should be evaluated.
Report Sentence
The mass shows peripheral heterogeneous enhancement with extensive central necrosis, consistent with an aggressive sarcoma pattern.
On FDG PET-CT, UPS shows intense FDG uptake (SUVmax typically >8-15). It is one of the types showing highest FDG uptake among retroperitoneal sarcomas. Necrotic areas show low uptake. PET-CT is valuable for staging, treatment response assessment, and recurrence detection. High SUVmax reflects high-grade tumor and correlates with poor prognosis.
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Markedly increased FDG uptake (SUVmax: ___) is seen in the retroperitoneal mass, consistent with high-grade sarcoma (UPS).
Criteria
Dominant storiform pattern and pleomorphic cells; most common type
Distinct Features
Heterogeneous enhancement, prominent necrosis, minimal myxoid component
Criteria
Myxoid matrix comprises >50% of the tumor
Distinct Features
Markedly homogeneous high signal on T2 (myxoid), less necrosis, relatively better prognosis
Criteria
Contains prominent multinucleated giant cells
Distinct Features
Indistinguishable from conventional type on imaging, histological diagnosis
Distinguishing Feature
Leiomyosarcoma may show IVC involvement/intraluminal extension; IVC involvement not expected in UPS
Distinguishing Feature
SFT shows low T2 signal (collagen), intense homogeneous enhancement; UPS heterogeneous T2, prominent necrosis
Distinguishing Feature
Lymphoma shows homogeneous mild enhancement and encases vessels without occlusion; UPS shows heterogeneous intense enhancement and necrosis
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
3-monthUPS is a high-grade, aggressive sarcoma. Wide surgical resection (R0) is essential. Adjuvant radiotherapy improves local control. Chemotherapy (doxorubicin + ifosfamide) is considered for advanced or inoperable disease. Local recurrence rate is 30-50% and 5-year survival is 30-50%. Close follow-up (3-monthly CT) is required for the first 2-3 years. Most common metastatic sites: lung (50%), liver (20%), bone (10%).
UPS is an aggressive tumor, and complete surgical resection is the primary treatment. Local recurrence rates are high (50-70%). Radiotherapy is added for local control. Systemic chemotherapy may be used palliatively.