FIBROOSSEOUS PSEUDOTUMOR OF THE DIGIT - A COMPARISON TO MYOSITIS-OSSIFICANS BY LIGHT-MICROSCOPY AND IMMUNOHISTOCHEMICAL METHODS

Fibro-osseous pseudotumor of the digit is an unusual cutaneous process characterized histologically by a fibroblastic proliferation admixed with reactive/metaplastic osteoid formation. The osteoid formation can be florid and immature, mimicking the appearance of malignant osteoid -forming neoplasms. Fibro-osseous pseudotumor of the digit has histolo gic and clinical features in common with myositis ossificans. This has led many to consider the two to be synonymous. We studied three cases of fibro-osseous pseudotumor, compared to five cases of myositis ossi ficans, using routine light microscopy and a battery of immunohistoche mical stains. Both entities displayed a ''zoning'' pattern of immature spindled areas admired with more mature areas having osteoid metaplas ia. This was more pronounced in myositis ossificans. In each lesion, t he spindle cells stained positively for vimentin and actin. CD34 and F actor VIII highlighted the vasculature. No stromal staining for MAK-6 (cytokeratin) or S-100 was identified. Ki-67, a proliferation marker, showed positive staining of the stromal cells in both lesions, which w as strongest in the immature spindled areas. The immunohistochemical a nd histologic similarities of the lesions support fibro-osseous pseudo tumor of the digit being a cutaneous variant of myositis ossificans.