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| May 2007
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Plexiform fibrohistiocytic tumour was first described by Enzinger and Zhang in 1988 and is characterized by fibrohistiocytic cytomorphology and a multinodular growth pattern. It is a rare mesenchymal neoplasm of children, adolescents, and young adults characterized by low-grade malignant behaviour. The tumour is prone to recur locally and occasionally to metastasize regionally and systemically. The most common location is the upper extremities, followed by the lower extremities, trunk, head, and neck. The tumour typically presents as a painless dermal or subcutaneous mass, which slowly grows for a period of months or years. Gross features: Grossly, the tumour is usually described as being gray-white and firm and ranging in size from 0.5 to 8.0 cm. Microscopic features: Microscopically, extensions of fibrous tissue into the surrounding fat identified on low-power is quite characteristic of Plexiform fibrohistiocytic tumour. Most cases show a rim of normal dermis above the lesions and the overlying epidermis is unremarkable. The nodular component consist of nests of mononuclear histiocytic cells that are often admixed with osteoclast-like multinucleated giant cells, showing no nuclear atypia or necrosis . The clusters of mononuclear histiocytic cells are circumscribed by short fascicles of fibroblast-like cells that intersperse between collagen bundles. Most tumours display mitotic activity, frequently <3 mitoses/10 high-power fields, and rarely the tumour may display atypical mitoses. Based on proportion of fibroblast-like spindle cells and mononuclear histiocytic cells there are 3 histologic subtypes: i) fibrohistiocytic, ii) fibroblastic, iii) mixed. The fibrohistiocytic type consists of nests of histiocytic cells interspersed with multinucleated giant cells. The fibroblastic type consists of spindle fibroblast-like cells arranged in a plexiform pattern resembling fibromatosis. There may be areas of haemorrhage which is a useful diagnostic feature. Other rare features include granular cell change, hyalinization, osseous metaplasia and myxoid changes. Vascular invasion / intravascular growth has been observed in some cases.
Immunohistochemistry: Vimentin-Positive ; Alpha-1-antitrypsin & Alpha-1-antichymotrpsin- Positive Smooth Muscle Actin - Positive (spindle fibroblast-like cells are commonly positive for smooth muscle actin and are occasionally positive for CD34). CD68 - stained mainly the multinucleated giant cells and, to a lesser extent, mononuclear histiocyte-like cells and, occasionally, fibroblast-like cells. Desmin & S100 protein- Negative. Electron microscopy:Show features of undifferentiated cells, myofibroblasts, and fibroblasts. Differential Diagnosis: Dermatofibroma: Older patients. Foam cells are present. No evidence of plexiform pattern and characteristic nodules present in Plexiform fibrohistiocytic tumour. Tumours with a prominent plexiform pattern such as plexiform neurofibroma(S-100 positive, and usually associated with neurofibromatosis), plexiform schwannoma, fibrous hamartoma of infancy, cellular neurothekeoma, desmoid fibromatosis, and nodular fasciitis. Other tumours included in the differential diagnosis are atypical fibroxanthoma, pleomorphic MFH and giant cell sarcoma of soft part.
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