Soft tissue sarcoma (UPS — Undifferentiated Pleomorphic Sarcoma, formerly MFH — Malignant Fibrous Histiocytoma) is the most common high-grade soft tissue sarcoma in adults. It accounts for approximately 20-25% of all soft tissue sarcomas. Most common between ages 50-70 with a slight male predominance. Lower extremity (thigh) is the most common location, and the vast majority of lesions are deep to fascia. It typically presents as a large (>5cm), heterogeneous, aggressive mass. Areas of necrosis and hemorrhage are common. Risk of hematogenous metastasis (especially lung) is high.
Age Range
50-80
Peak Age
65
Gender
Male predominant
Prevalence
Uncommon
UPS is a high-grade malignant mesenchymal neoplasm showing no specific line of cell differentiation. Diagnosis is made after exclusion of other specific sarcoma types (liposarcoma, leiomyosarcoma, synovial sarcoma, etc.) — the 'exclusion diagnosis' principle applies. The tumor contains pleomorphic spindle and round cells, multinucleated giant cells, and variable amounts of collagenous stroma. This heterogeneous cellular composition explains the heterogeneous signal pattern on MRI — different cell types, necrosis, hemorrhage, and fibrotic areas give different T1/T2 signals. Necrotic areas result from inadequate vascularization and appear as non-enhancing areas on CT/MRI. Hemorrhagic components show different signal characteristics with methemoglobin (T1 hyperintense) and hemosiderin (T2 hypointense) accumulation. The aggressive growth pattern of the tumor manifests as invasion of surrounding structures, compartment transgression, and encasement of neurovascular structures.
A subfascial mass >5cm in size with markedly heterogeneous enhancement containing non-enhancing necrotic areas is the most important imaging finding for soft tissue sarcoma. This combination reflects aggressive biological behavior.
On T1-weighted sequences, UPS shows heterogeneous signal. Solid tumoral component shows intermediate-to-low signal, hemorrhagic areas are T1 hyperintense (methemoglobin), fibrous areas appear hypointense. Necrotic areas show low signal. This heterogeneous T1 pattern is typical for an aggressive soft tissue mass.
Report Sentence
On T1-weighted sequences, a deep-seated heterogeneous mass is seen with T1 hyperintense foci consistent with hemorrhagic areas.
On T2-weighted sequences, UPS shows markedly heterogeneous signal. Solid viable tumor shows intermediate-to-high T2 signal, necrotic areas show very high T2 signal (fluid), hemorrhage shows variable signal depending on stage (acute hemosiderin hypointense, subacute hyperintense), fibrous areas show low T2 signal. Peritumoral edema may appear as a T2 hyperintense halo.
Report Sentence
On T2-weighted sequences, the mass demonstrates markedly heterogeneous signal containing areas of necrosis and hemorrhage.
On STIR sequences, UPS generally shows high signal; after fat suppression, solid tumoral component and necrotic areas appear markedly hyperintense. Hemosiderin-containing areas may show low signal. STIR is more sensitive than T2 in evaluating tumor margins and peritumoral edema, and is critically important in determining bone and soft tissue extension.
Report Sentence
On STIR sequences, the mass shows high signal, and peritumoral edema and anatomic compartment relationship have been evaluated.
On post-contrast images, UPS shows heterogeneous enhancement. Viable solid tumoral component demonstrates avid enhancement while necrosis and cystic degeneration areas show no enhancement. This 'negative enhancement' pattern (non-enhancing areas) indicates aggressive tumor necrosis. Peripheral enhancement pattern may also be seen. Fat-suppressed post-contrast sequences are the most sensitive technique for enhancement evaluation.
Report Sentence
On post-contrast images, the mass demonstrates heterogeneous enhancement with central necrotic areas showing no enhancement.
On DWI, solid viable components of UPS show diffusion restriction — high DWI signal and low ADC values. Necrotic areas may show low signal or T2 shine-through on DWI. ADC values are inversely proportional to tumor grade and cellularity; low ADC (<1.0 × 10⁻³ mm²/s) suggests high-grade sarcoma. DWI is also valuable in monitoring treatment response — ADC increase is expected after successful treatment.
Report Sentence
On DWI, diffusion restriction (low ADC) is seen in solid components of the mass, consistent with high cellularity and malignant process.
On CT, UPS appears as a large, heterogeneous, well or poorly defined soft tissue mass. On non-contrast CT, low-to-intermediate density with high density in areas of internal hemorrhage, calcification rare. Post-contrast heterogeneous enhancement with non-enhancing central necrotic areas. CT is particularly valuable for evaluating bone invasion and lung metastasis screening.
Report Sentence
On CT, a deep-seated, heterogeneously enhancing soft tissue mass is seen with central necrotic areas.
On PET-CT, UPS shows avid FDG uptake (SUVmax usually >5-8). Necrotic areas show low FDG uptake (photopenic areas). PET-CT is valuable in staging, treatment response assessment, and recurrence detection. PET-CT guides identification of the most metabolically active region before biopsy.
Report Sentence
On PET-CT, avid FDG uptake is seen within the mass, consistent with high metabolic activity and high-grade malignant process.
Criteria
No specific cell differentiation, pleomorphic cells, requires exclusion of other sarcoma types, diagnosed by IHC
Distinct Features
Most common subtype. Usually in older adults. Large mass deep to fascia. High grade, metastasis risk 30-35%. 5-year survival 60-65%. Variable chemotherapy response.
Criteria
Prominent myxoid component + fibrosarcoma features, low-to-high grade spectrum, usually superficial dermal/subcutaneous location
Distinct Features
Very bright T2 signal (myxoid matrix). 'Tail sign' — tail-like extensions of tumor along fascial planes. High local recurrence rate (50-60%). Solid component increases with grade and prognosis worsens. More common in elderly patients.
Criteria
Develops within prior radiotherapy field, >3 year latent period, located within radiation field, different histologic type (independent from prior tumor)
Distinct Features
Most common after breast, cervix, or lymphoma radiotherapy. Average latent period 8-12 years. Usually high grade. Location overlapping with prior treatment field is diagnostic clue. Typical sarcoma features on MRI. Prognosis similar to sporadic sarcomas.
Distinguishing Feature
Liposarcoma (well-differentiated/dedifferentiated) contains fat signal (T1 hyperintense fat component); UPS contains no fat component, entirely solid heterogeneous mass
Distinguishing Feature
Metastasis usually has known primary malignancy history, tends to be multiple, marrow infiltration in bone metastasis; UPS usually solitary, in deep soft tissue
Distinguishing Feature
Schwannoma nerve-associated, target sign, split-fat sign, usually smaller and more homogeneous; UPS no nerve association, large, heterogeneous, necrotic
Distinguishing Feature
Desmoid tumor contains low T2 signal areas (collagen), necrosis rare, homogeneous-moderate enhancement, usually younger patients; UPS heterogeneous T2, marked necrosis, aggressive enhancement
Urgency
urgentManagement
wide surgical excision with negative margins; neoadjuvant/adjuvant radiotherapy; chemotherapy for high-grade/metastatic; multidisciplinary sarcoma tumor boardBiopsy
NeededFollow-up
MRI every 3-4 months for first 2-3 years, then every 6 months; chest CT for lung metastasis surveillance; lifelong follow-up recommendedUPS is a high-grade malignant tumor and should be managed at a multidisciplinary sarcoma center. Wide surgical resection (negative surgical margins) is the primary treatment. Neoadjuvant/adjuvant radiotherapy is frequently applied for local control. Chemotherapy is used for high-grade, large tumors and metastatic disease. Metastasis risk is 30-35%, most commonly lung metastasis. Biopsy should be performed as MRI-guided core biopsy, tru-cut preferred. The biopsy tract should be excised during surgery (tumor seeding risk). 5-year overall survival is 60-65%.
Treatment is wide surgical excision + neoadjuvant/adjuvant radiotherapy. As a high-grade tumor, it has high risk of local recurrence and distant metastasis (lung most common). Pre-surgical MRI should assess neurovascular involvement, compartmental relationships, and surgical margin planning. 5-year survival is approximately 50-65%.