OFT almost exclusively affects adults (mean age near 50 years) with rare examples in children.9 10
Men are affected more commonly than women. Most patients present with a small, painless, well-defined, often lobulated subcutaneous mass which involves the extremities. Grossly OFT is usually well circumscribed, covered by a thick fibrous pseudocapsule. Most measure 3–5 cm, but occasional lesions are 15 cm or larger.11
Microscopically, the majority are located in the subcutaneous tissue but some are attached to tendons, fascia or involve the underlying skeletal muscle. The tumour is composed of uniform round ovoid to spindle-shaped cell arranged in nests or cords and deposited in the variably myxoid and collagenous stroma. In approximately 80% of cases, there is an incomplete shell of lamellar bone found at the periphery of the nodules, but up to 20% of cases are non-ossifying.12 13
Immunohistochemically, the cells are positive for vimentin and express S100 protein in about 70% of cases the cells may also express leu7, neuron specific enolase and glial fibrillar acidic protein. The histogenesis of this tumour is controvercial but Min et al14
suggested the cell have the ultrastructural features of myoepithelial cells. The differential diagnosis includes benign and malignant epitheliod nerve sheath tumours (epithelioid neurofibroma, epithelioid schwannoma, epithelioid malignant peripheral nerve sheath tumour), chondroid syringoma (cutaneous mixed tumour), myxoid chondrosarcoma and epithelioid smooth muscle tumours.
Behaviour of the OFT varies. The vast majority of these tumours are histologically benign. Not unexpectedly most of these pursue a benign clinical course. However, it has been noted that on rare occasion even histologically typical tumours may locally recure or metastasize. This is true for our case. Yoshida and colleagues2
described a case lacking malignant features but which locally recurred, metastatised and ultimately killed the patient. some of these tumours have atypical histologic features including high nuclear grade, high cellularity and mitotic activity of >2 MF/50 HPF.14
It is suggested that tumours with these atypical features should be regarded as sarcomas with significant potential for metastasis. But since cases that deviated from typical OFT had a metastatic rate (6%) that was similar to typical tumours (4%), Folpe and Weiss13
opted to avoid the term atypical OFT and considered those cases within the general category of OFT.
- OFT is considered to be of mesenchymal origin. Most of these tumours pursue a benign clinical course but on rare occasion even histologically typical tumours may locally recure or metastasize, primary in the lung even several years after surgical removal. Because of rarity, the recognition of this entity is important.
- OFT is a soft tissue tumour of intermediate malignancy of uncertain type.
- There is no consensus about appropriate treatment at present.