Fibroblast-myofibroblast differentiated soft tissue tumors

1. Nodular Fasciitis

Definition

• Benign, non-cancerous soft tissue growth.
• Also called pseudosarcomatous fasciitis (mimics sarcoma microscopically).
• Occurs in young adults (20–40 yr).
• Sites: extremities, trunk, head + neck region.

Pathogenesis

• All cases contain fusion genes involving USP6.
• Most common: MYH9-USP6 fusion.

Morphology

• Generally small (< 3 cm), can be larger.
• Wavy spindle-shaped tumor cells (fibroblasts / myofibroblasts).
• “Tissue culture-like” pattern.
• Loose mucoid/collagenous matrix with ↑ cellularity.
• Extravasated RBCs.
• Mitotic activity (high — reason it mimics sarcoma).

Clinical Features

• Self-limited disease → regresses + disappears.
• May be asymptomatic or mildly painful.
• Rapid growth raises concern for malignancy.
• Simple excision is curative.

2. Fibromatosis (Desmoid Type)

Definition

• Benign fibroblastic / myofibroblastic tumor arising in deep soft tissue.
• Locally aggressive with infiltrative growth.
• NO metastatic potential.
• Behavior: “intermediate” — between benign + malignant.

Sites

• Abdominal region (intra-abdominal, mesentery, colon-adjacent).
• Chest wall + breast.
• Extremities, shoulder girdle.

Types

• Superficial fibromatosis (palmar / Dupuytren contracture, plantar Ledderhose, Peyronie).
• Deep / desmoid type: β-catenin mutated cell accumulation (CTNNB1 mutation) or APC mutation.

Associations

• Gardner syndrome = familial adenomatous polyposis (FAP) + fibromatosis (desmoid tumors) + osteomas.
  • APC gene mutation.

Morphology

• Well-differentiated fibroblasts + myofibroblasts.
• Long fascicular arrangement of spindle-shaped cells.
• Pale eosinophilic cytoplasm.
• Often arise in muscular fascia.
• Histology can mimic fibrosarcoma but lacks atypia.

Clinical Features

• Very aggressive locally with infiltrative pattern.
• Frequent recurrence after resection.
• NO metastatic potential.
• Tx: prompt surgical excision with wide margins; radiation + tamoxifen + NSAIDs adjuncts.

3. Summary Table

Tumor	Behavior	Site	Key features
Nodular fasciitis	Benign, self-limited	Extremities, trunk, H&N	Pseudosarcomatous, young adults, USP6 fusion (MYH9-USP6), tissue culture-like spindle cells, extravasated RBCs, simple excision curative
Fibromatosis (desmoid)	Locally aggressive; NO mets	Abdomen, chest wall (breast), extremities	β-catenin (CTNNB1) / APC mutation; Gardner syndrome (FAP + osteoma + desmoid); long fascicles of bland spindle cells; wide excision; frequent recurrence

💡 High-yield: Nodular fasciitis = benign pseudosarcoma in young adults; USP6 fusion (MYH9-USP6); “tissue culture-like” spindle cells in loose mucoid matrix + extravasated RBCs; self-limited — simple excision curative. Desmoid fibromatosis = β-catenin (CTNNB1) or APC mutation; locally aggressive infiltrative, NO mets; abdominal/chest wall/breast; Gardner syndrome = FAP + osteoma + desmoid; wide excision often recurs (no metastasis).