Myxoid soft tissue tumors

General characteristics and MRI features

Histologically, these tumors consist of a background mucoid material with various cell types. It is the myxoid stroma that determines how the lesion appears on scans, which is why different myxoid tumors share a similar radiological appearance.

MRI is the most informative diagnostic method (CT and ultrasound yield non-specific findings). Characteristic MRI features of myxomas include:

• T1-weighted images (T1WI): hypointense (low) signal.
• T2-weighted images and STIR: hyperintense (high) signal, making the tumor visually resemble a cyst.
• Structure: can be homogeneous or heterogeneous due to thin fibrous septa and cystic areas.
• Surrounding changes: soft tissue edema is frequently observed. A rim of fat may be visualized at the poles — the split-fat sign — reflecting the infiltrative nature (muscle atrophy).

To differentiate a myxoma from a true cyst, intravenous contrast administration is mandatory: enhancement with a paramagnetic agent confirms the non-cystic, solid nature of the lesion.

Differential diagnosis

Due to their cyst-like appearance, myxoid tumors must be differentiated from a number of lesions of synovial and periarticular origin. The differential diagnosis includes:

1. Ganglia (tendon cysts).
2. Synovial cysts.
3. Bursitis.

Additionally, there are non-neoplastic mimics that clinically disguise themselves as mass lesions, but imaging modalities allow for a more accurate diagnosis compared to true neoplasms.

Soft tissue sarcomas (STS)

Sarcomas are rare malignant tumors of mesenchymal origin. The most common type in adults is undifferentiated pleomorphic sarcoma (UPS), frequently featuring central necrosis and hemorrhage.

MRI poorly differentiates benign from malignant processes, thus a biopsy is required. However, there are clear MRI features suspicious for malignancy:

• Deep location (beneath the fascia).
• Large size (risk factor: size $\ge$ 5 cm).
• Heterogeneous structure.
• Presence of necrosis (areas without contrast enhancement).

Synovial sarcoma

A distinct entity is synovial sarcoma (Synovial Sarcoma). Contrary to its name, less than 10% of these tumors are intra-articular. They typically arise in the lower extremities in individuals aged 15–40 years.

Synovial sarcoma on T2-WI is characterized by the "triple signal" sign (triple signal), which occurs in 35% of cases. It represents the presence of areas of high, low, and isointensity (relative to adipose tissue), reflecting a combination of necrosis/fluid, fibrosis/calcifications, and solid tissue. The tumor can grow slowly and appear homogeneous, carrying a high risk of misdiagnosis as a benign lesion.

MRI evaluation checklist for a mass lesion

Imaging methods do not replace histology, but are strictly mandatory for local staging. The physician must evaluate:

• Localization: relationship to the investing fascia.
• Size: measurements in all three planes.
• Signal: search for specific features, hemorrhage, necrosis.
• Margins: well-defined or ill-defined, presence of edema signal in surrounding tissues, and adjacent tumor nodules.
• Involvement of adjacent structures: neurovascular bundles, bones, joints, regional lymph nodes.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mazabraud syndrome and the genetics of bone fibrous dysplasia
• Split-fat sign: mechanism of formation
• Undifferentiated pleomorphic sarcoma (UPS) in MRI practice
• Juxtarticular myxomas of the knee joint
• Features of rhabdomyosarcoma in young children

Other topics in the "Musculoskeletal System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.