Soft Tissue Mass

Christopher G. Anton, MD

DIFFERENTIAL DIAGNOSIS

Common

Ganglion Cyst
Lipoma
Hematoma
Vascular Malformation
Hemangioma
Fat Necrosis

Less Common

Rhabdomyosarcoma (RMS)
Myositis Ossificans
Neurofibroma (NF)
Synovial Sarcoma

Rare but Important

Extraosseous Ewing Sarcoma (EOES)
Fibromatosis
Fibrosarcoma (FS)
Malignant Peripheral Nerve Sheath Tumor
Lipoblastoma
Liposarcoma
Other Sarcomas
Congenital or Infantile

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Some soft tissue masses can be diagnosed by clinical exam
- Lipoma: Superficial & doughy by palpation
- Ganglion cyst: Transilluminates and near joint
- Others need further evaluation by radiographs and then MR
MR will help differentiate determinate from indeterminate lesions
- Determinant lesions: Neurofibroma, vascular malformations, hematoma, lipoma
  - Excision biopsy or monitoring
- Indeterminate: Underlying pathology is uncertain
  - Needle biopsy to determine management

Helpful Clues for Common Diagnoses

Ganglion Cyst
- MR: Homogeneous hyperintense T2 signal cystic mass, peripheral enhancement
- Communicates with joint space or tendon sheath
Lipoma
- May represent up to 1/3 of all soft tissue masses
- May appear lucent on radiographs
- MR: Follows subcutaneous fat signal on all pulse sequences
  - Hint: Use fat-suppression sequences
  - May have thin septations
  - Intramuscular lipomas may appear more complex with infiltration and poor definition
Hematoma
- MR: Hemorrhagic products
  - Hyperintense T1 and hypo- to hyperintense on T2 with “blooming” on gradient echo sequences
- Must use caution; may be difficult to differentiate from soft tissue sarcoma with hemorrhage
Vascular Malformation
- Present at birth
- Venous: Low-flow lesion on gradient echo MR images with thrombi, phleboliths, and diffuse contrast enhancement
  - May see calcified phleboliths on radiographs
- Lymphatic: Low-flow lesion on gradient echo MR images with septal enhancement and often fluid-fluid levels
- Arteriovenous: High-flow lesion on gradient echo MR images; tangle of vessels without enhancing soft tissue mass
Hemangioma
- True neoplasm
- Small or absent at birth, rapid growth over 1st several months, involutes over months to years
- MR: Hyperintense on T2, vascular flow voids (high flow on GE images), intense post-contrast enhancement
  - May have fatty component when involuting
- Ultrasound: High vessel density (> 5 vessels/cm²)
- Strawberry skin discoloration if superficial or bluish in deeper hemangiomas
Fat Necrosis
- Most commonly follows trauma (may not recall) and over bony protuberances
- Small, stellate, spiculated, linear with lack of soft tissue mass
- May possess peripheral or irregular marginal contrast enhancement

Helpful Clues for Less Common Diagnoses

Rhabdomyosarcoma (RMS)
- Most common soft tissue sarcoma in children
- Embryonal RMS: 60-70% of childhood RMS
  - Typically < 15 years old; more common in GU and head and neck
- Alveolar RMS: Adolescents; most common in extremity, trunk, perianal/perirectal
- Alveolar and extremity RMS tend to have worse prognosis
Myositis Ossificans
- Ring-like peripheral calcifications
Neurofibroma (NF)
- Frequently multiple and seen in neurofibromatosis type 1
- May be sporadic and solitary
- “Target” sign or “bag of worms”
Synovial Sarcoma
- 2nd most common sarcoma in childhood, 15-35 years old
- Calcifications in 1/3 can appear nonaggressive on MR, near joint; when extensive, improved prognosis

Helpful Clues for Rare Diagnoses

Extraosseous Ewing Sarcoma (EOES)
- May erode adjacent bone
Fibromatosis
- MR: Typically hypointense to skeletal muscle on T1 and hyperintense with areas of hypointensity on T2, may enhance
Fibrosarcoma (FS)
- Infantile FS < 5 years old; more common in lower extremities, heterogeneous enhancement, local recurrence
- Rarely metastasizes, better prognosis than adult FS
Malignant Peripheral Nerve Sheath Tumor
- Most commonly associated with NF1 (50%); more common in lower extremities
- Must consider malignant degeneration of neurofibroma if erodes bone, painful, rapid growth, and loss of “target” sign
Lipoblastoma
- Primarily < 3 years old, superficial arms and legs (deeper in lipoblastomatosis)
- MR: Composed of fat and myxoid tissue, hyperintense on T1, heterogeneous on T2
Liposarcoma
- Myxoid subtype more common, bright on T2 with heterogeneous enhancement
- Well-differentiated subtype; looks like lipoblastoma (but > 3 years old)
Other Sarcomas
- PPNET, epithelioid sarcoma, MFH, ASPS
Congenital or Infantile
- Infantile myofibromatosis, fibrous hamartoma of infancy, congenital-infantile fibrosarcoma, etc.

Image Gallery

Longitudinal ultrasound shows a well-defined anechoic fluid collection

in the volar aspect of the wrist. Connection to the joint or tendon sheath was not seen, but this is still likely a ganglion cyst.

Axial T2WI MR shows a bright hyperintense signal mass extending from the proximal tibia-fibia joint. The mass demonstrated peripheral enhancement (not shown), typical of a ganglion cyst.

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