Fibromatosis

Benign (non-metastasising) fibroblastic/myofibroblastic proliferation that behaves like a “bad neighbour”: infiltrative + locally aggressive + high local recurrence.

Imaging features

General pattern

• Mass-forming soft-tissue lesion, often along fascia/aponeuroses
• Encases structures more than it destroys; may cause distortion/tethering

Ultrasound

• Hypoechoic, solid, often ill-defined; variable vascularity
• "Clean" posterior acoustic shadowing

CT

• Soft-tissue attenuation mass, variable margins
• Intra-abdominal desmoid may encase mesenteric vessels/bowel without frank invasion

MRI

• T1: low–intermediate
• T2: variable (cellular = higher T2; collagenous = lower T2)
• Classic: band-like low-signal strands (collagen) on T2
• Enhancement: usually moderate–avid, often heterogeneous
• “Fascial tail” / aponeurotic extension can be seen (not exclusive)

Main subtypes

• Desmoid-type (deep fibromatosis): abdominal wall, mesentery, extra-abdominal (shoulder girdle, thigh, pelvic muscles)

• Palmar fibromatosis (Dupuytren): palmar aponeurosis → nodules/cords, finger flexion contracture

• Plantar fibromatosis (Ledderhose): medial plantar fascia nodules

• (Less exam-relevant but common): infantile/juvenile variants exist

Top differentials

• Fibrosis/scar: usually non-masslike architectural distortion; stable/contractile; less “fascial tail” mass behaviour
• Soft-tissue sarcoma: tends to be more heterogeneous, necrosis/haemorrhage, more destructive; metastasis risk
• Nodular fasciitis: rapid growth, often more oedematous/high T2; self-limited; clinical history helps
• Fibrosarcoma / low-grade fibromyxoid sarcoma: imaging overlap → biopsy if atypical/growing
• GIST/Lymphoma (intra-abdominal mass): location and enhancement pattern; desmoid often wraps/encases rather than obstructs early

Pearls

• Key behaviour: no metastases but high recurrence, can be morbid by encasement
• Association: FAP/Gardner (APC) → mesenteric desmoids (think post-op trigger)
• MRI clue: low-signal collagen bands within a variable T2 mass
• Interval growth vs stability (many are observed or treated non-operatively depending on site/symptoms)