Atypical Fibroxanthoma

Definition

Cellular cutaneous mesenchymal neoplasm with at least focal pleomorphism and lacking evidence of epithelial or melanocytic differentiation

This is a diagnosis of exclusion and the following must be ruled out in every case:
- Carcinoma
- Melanoma
- Smooth muscle tumor
- Nerve sheath tumor
- Pleomorphic malignant fibrous histiocytoma
Nearly always involves sun damaged skin of the elderly
- May also occur in irradiated areas
Involves dermis
- Usually abuts epidermis
- May ulcerate
- More than minimal extension into subcutaneous tissue is not allowed
Compressive margins
Vascular invasion not allowed
Cellular neoplasm composed of spindled to large pleomorphic cells
- Most common pattern is identical to pleomorphic malignant fibrous histiocytoma
  - Numerous large polygonal cells
  - Large bizarre nuclei
  - Numerous atypical mitotic figures
- Spindle cells may predominate
  - Pleomorphic cells should be present at least focally
- Xanthoma cells frequently present
Occasional findings:
- Hemosiderin
- Osteoclast-like giant cells
- Osteoid
- Clear cytoplasm
  - Rarely predominates
  - Lipid positive, glycogen negative
- Granular cytoplasm
  - Rarely predominates

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

Original posting: August 10, 2008
Last update December 31, 2008