Metaplastic Carcinoma of the Breast

Definition

Breast carcinoma exhibiting either a mesenchymal (or other non-epithelial) component or squamous differentiation

Must have a neoplastic component that is either squamous or non-epithelial
Carcinoma mixed with a mesenchymal or other non-epithelial component
- May exhibit obviously malignant stroma
  - May resemble pleomorphic MFH or fibrosarcoma
  - May exhibit heterologous differentiation
    - Usually osteosarcoma or chondrosarcoma
    - Less commonly glioma, melanoma, rhabdomyosarcoma, angiosarcoma or liposarcoma
- Stroma may be composed of bland spindle cells (spindle cell carcinoma)
  - p63 positive, often high molecular weight keratin positive
  - May contain fibroblasts and or myofibroblasts
  - May resemble nodular fasciitis or fibromatosis
- Mesenchymal component may be keratin positive in about half of cases
  - High molecular weight keratin is most often positive
- Epithelial component may be sparse
  - May be identified only by presence of keratin reactivity
    - Broad spectrum and high molecular weight keratins are useful
  - Squamous component may be cystic
Carcinomas with metaplastic squamous components
- May be pure squamous or mixed with ductal carcinoma
- Low grade adenosquamous carcinoma
  - May be considered as metaplastic carcinoma or as a distinct entity
Most have a component of ductal carcinoma
- There are rare reports of metaplastic carcinoma associated with lobular, medullary, mucinous and tubular carcinoma

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

Original posting: May 1, 2006
Updates: January 21, 2009