Ossifying Fibromyxoid Tumor

Definition

Tumor composed of lobules and cords of small bland cells in a myxohyaline background with frequent peripheral ossification

Grossly circumscribed
- May have microscopic micronodular extension
Peripheral layer of lamellar bone in most cases
- May be focal
- Occasionally within tumor
- Rarely extensive
- Occasional cases have only fibrous capsule
Lobules and cords of bland cells
- Small, usually round to oval epithelioid cells, occasionally spindled
- Bland vesicular nuclei
- Cytoplasm pale to eosinophilic
- Mitotic figures rare
- Nucleoli small to absent
Stroma myxoid to hyaline
- Prominent thin walled blood vessels
Predominantly subcutaneous, occasionally intramuscular
- Occasionally involving skin
Features associated with recurrence or metastasis
- High cellularity
- High nuclear grade
- Mitotic count >2/50 hpf
- Infiltrative growth
- The diagnosis of malignant ossifying fibromyxoid tumor should only be made in the presence of areas of typical tumor or with a prior history of typical tumor
Other unusual features
- Mucinous microcysts
- Vascular invasion
- Necrosis
- Lack of myxoid stroma
- Microcalcifications
- Associated epidermoid cysts
- Chondroid differentiation with binucleate chondrocytes
- Multinucleated giant cells

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