🩺 Introduction
Soft tissue sarcomas (STS) are a heterogeneous group of malignant mesenchymal tumors. Imaging plays a crucial role in detection, characterization, local staging, biopsy planning, and treatment follow-up.
🖼️ Imaging Modalities
1️⃣ Radiography
- Often normal or nonspecific.
- May show:
- Soft tissue mass with or without calcifications.
- Bone erosion or periosteal reaction if adjacent bone involved.
2️⃣ Ultrasound
- First-line for superficial lesions.
- Findings:
- Solid hypoechoic mass with internal vascularity.
- Useful for guiding biopsy.
- Limitation: cannot define deep extension.
3️⃣ CT
- Useful for deep, retroperitoneal, thoracic, pelvic STS.
- Shows extent, density, calcification, necrosis, adjacent organ involvement.
- Helpful for lung metastasis evaluation.
4️⃣ MRI (modality of choice)
- Gold standard for local staging.
- Key findings:
- Large, heterogeneous soft tissue mass.
- T1: Iso- to hypointense to muscle.
- T2: Hyperintense, often heterogeneous.
- STIR/T2 fat sat: Highlights edema and extent.
- Post-contrast: Heterogeneous enhancement, necrotic/cystic areas.
- Fascial plane involvement, neurovascular encasement, skip lesions.
⚡ Key MRI Signs
- Split fat sign → preserved fat rim around benign lesions; often lost in sarcomas.
- Fascial tail sign → enhancing fascial extensions; suggests aggressive nature.
- Perilesional edema → can be seen in sarcomas but also in benign aggressive lesions.
🧩 Role of Imaging
- Diagnosis & Characterization → MRI features suggest malignancy but biopsy is required.
- Local Staging → Tumor size, compartments, fascial planes, neurovascular involvement.
- Biopsy Planning → Imaging guides the safest biopsy tract (must be placed along surgical approach).
- Follow-up → MRI for local recurrence, CT chest for metastasis (most common site = lung).
🧠 Teaching Pearls
- MRI is the modality of choice for local staging.
- Always report: size, margins, compartments, neurovascular involvement, bone invasion, skip lesions.
- Chest CT is mandatory for staging (lung mets).
- Imaging + biopsy = final diagnosis.
📊 Benign vs Malignant Soft Tissue Tumors – MRI Features
| Feature | Benign Tumors | Malignant (Soft Tissue Sarcomas) |
|---|---|---|
| Size | Usually < 5 cm | Often > 5 cm |
| Margins | Well-defined, smooth | Ill-defined, infiltrative |
| Signal on T1 | Iso-/hypointense to muscle, homogeneous | Hypo- to isointense, heterogeneous |
| Signal on T2/STIR | Homogeneous hyperintense | Markedly hyperintense, heterogeneous with necrosis/hemorrhage |
| Enhancement | Homogeneous, mild to moderate | Heterogeneous, irregular, strong |
| Perilesional edema | Minimal or absent | Often present (may mimic inflammation) |
| Fat plane preservation | Preserved (“split fat sign”) | Lost; invasion of fascial planes |
| Fascial involvement | Rare | Fascial tail sign common |
| Neurovascular encasement | Absent | May be present |
| Bone involvement | Rare (pressure erosion) | Cortical invasion, destruction |
| Growth pattern | Slow, stable | Rapid, progressive |
| Recurrence | Rare after excision | Common, needs follow-up |
- Size > 5 cm, deep location, heterogeneous enhancement, and fascial/neurovascular invasion strongly suggest sarcoma.
- Benign lesions (e.g., lipoma, hemangioma, neurogenic tumors) usually have smooth margins and respect fascial planes.
- Always combine MRI features with clinical findings + biopsy for definitive diagnosis.
✅ Conclusion
Soft tissue sarcomas are rare but aggressive tumors. MRI provides excellent local staging, CT complements for thoracic and retroperitoneal evaluation, and both are essential for management planning. Radiologists play a key role in the diagnosis-to-treatment pathway.
