Plexiform fibrohistiocytic tumor: clinicopathologic analysis of 22 cases.

Twenty-two cases of plexiform fibrohistiocytic tumor were reviewed to perform a clinicopathologic correlation with the behavior of the neoplastic entity. The tumor arises more frequently in children, adolescents, and young adults (mean age of presentation, 14.6 years), with strong female predilection (F:M, 6:1). It involves preferentially the upper extremity (64%), especially the fingers, hand, or wrist (45%). Most patients present with a small (average size, 2.5 cm; range, 0.5-8 cm) painless mass that slowly enlarges for months to years. All tumors involve subcutaneous adipose tissue, with extension into the dermis (19%), skeletal muscle (14%), or both (14%). Grossly, the tumors characteristically are poorly circumscribed and of firm consistency. Histologically, they are characterized by a plexiform proliferation of mononuclear histiocyte-like cells, multinucleated osteoclast-like cells, and spindle fibroblast-like cells in variable proportions and have three distinct growth patterns: fibrohistiocytic (36% of tumors), fibroblastic (32%), and mixed (32%), depending on the predominant cell type. Cellular atypia and pleomorphism are usually absent or minimal. Most tumors (78%) display mitotic activity, frequently <3 mitoses/10 high-power fields, and only 14% of the lesions display atypical mitoses. Vascular invasion was seen in only one tumor. Immunohistochemically, all tumors evaluated reacted with antibodies to CD68 that stained mainly the multinucleated giant cells and, to a lesser extent, mononuclear histiocyte-like cells and, occasionally, fibroblast-like cells. Less frequently, staining with antiactin antibodies was observed, restricted mainly to spindle cells. All nine tumors examined had a diploid DNA content. According to latest follow-up data (average period, 3.6 years) from 16 patients, 13 (82%) were alive with no evidence of disease (average, 3.6 years), 1 (6%) was alive with metastatic disease (follow-up, 2.3 years), 1 (6%) was alive with a stable pulmonary nodule of unknown nature (follow-up, 1.75 years), and 1 (6%) had died of disease 3 years after local recurrence and regional lymph node and pulmonary metastases developed. Two patients (12.5%) had local recurrence, 1 (6%) had regional lymph node metastasis, and 3 (19%) had pulmonary metastases. No proven association between clinicopathologic features and outcome was identified. In conclusion, plexiform fibrohistiocytic tumor is a rare mesenchymal neoplasm of young persons characterized by low-grade malignant behavior and is prone to recur locally and occasionally to metastasize regionally and systemically.