Myofibroblastoma of the Breast

Definition

Benign circumscribed spindle cell neoplasm of the breast exhibiting myofibroblastic differentiation

Fascicles of spindle cells partially separated by bands of hyalinized collagen
- Haphazard short fascicles of cells
- Uniform bland short spindle cells
  - Oval nuclei
  - Slightly irregular nuclear membrane
  - 1 or 2 small nucleoli
  - Occasional nuclear grooves
  - Moderate amount of pale to eosinophilic cytoplasm
- Infrequent mitotic figures (0-2/10 hpf)
Collagen bundles range from sparse to prominent
- Often arranged as thin strips
- Stroma occasionally myxoid
Sharply circumscribed
- Surrounding pseudocapsule
- No epithelial elements within lesion
Variable features
- Fat lobules occasional to prominient
- Rare multinucleated cells in some cases
- One case with focal cartilaginous differentiation
Variable reactivity for desmin, actin and CD34
It has been proposed that in the breast, myofibroblastoma forms a spectrum with spindle cell lipoma and solitary fibrous tumor (Magro 2002)
Similar lesion has been reported as "mammary-type myofibroblastoma of soft tissue" (McMenamin 2001)

Richard L Kempson MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342