= mass-forming fibrous proliferation

Age: 2^nd–4^th decades (most common)

Pathogenesis: unknown: ? reactive / inflammatory process

Histo: immature fibroblasts + variable amount of mature birefringent collagen

Types:

1. subcutaneous [3–10 x more common than (b) or (c)]
   • well-defined margins
2. intramuscular
   • typically larger + deeper in location
   • ill-defined margins with extension along fascial planes
3. intermuscular (fascial)

Location: head & neck (15–20%)

Size: 0.5–10 cm

CT:

• homogeneous mass with attenuation similar to fluid
• ± erosion of underlying bone

MR:

• heterogeneous mass usually slightly hyperintense relative to muscle on T1WI
• hyperintense relative to muscle on T2WI
• ± marked enhancement