44-year-old woman with left-sided breast mass   

Findings and diagnosis

Microscopic findings

A breast biopsy showed fascicles of spindle cells infiltrating around normal ducts and lobules. Mitoses are absent or rare. Lymphocyte aggregates are seen at the periphery. The lesion has irregular borders. Spindle cells do not demonstrate dysplastic features. No giant cells are seen. Normal, benign breast ducts are seen at the periphery. Beta-catenin shows nuclear positivity. CD34, desmin, and pancytokeratin are negative.

Differential diagnosis

  • Low grade fibromatosislike metaplastic carcinoma 
  • Pyoderma gangrenosum, acne, and suppurative hidradenitis (PASH) syndrome
  • Nodular fasciitis 
  • Fibromatosis 

Diagnosis: Fibromatosis