Infantile Fibromatosis

Definition

Bland fibroproliferative process that invades skeletal muscle, occurring age 8 and under, with two types:
- One type identical to adult desmoid fibromatosis
- Diffuse mesenchymal type, often containing fat

Limited to patients 8 years of age or under
- Some studies use a limit of 10 years
Infiltration of skeletal muscle
- Involvement may be superficial
Usually involves head, neck or thigh
Two patterns
- Immature or diffuse pattern
  - Uniform bland cells in myxoid stroma
    - Nuclei round or oval to spindled
    - Scant cytoplasm
  - Infiltrates between muscle fibers
    - Many muscle fibers remain intact
    - Peripheral inflammation may be present
    - May surround nerves
    - May invade bone, especially in head and neck
  - Fat is often present
    - When prominent, the term lipofibromatosis is used
- Adult type is identical to adult extra-abdominal desmoid fibromatosis