Fibrosarcoma, also known as mesenchymal cell tumor or fibroblastic malignancy, is a rare tumor of malignant fibroblasts that usually occurs in the third to fifth decade of life. It generally presents as a painless, enlarging, poorly defined mass in the soft tissues. The lower extremities are a site of predilection, especially the thigh and knee, but the tumor can be found on the trunk, the upper extremities, and the head and neck. 

Fibrosarcomas usually arise deep to the subcutis, from fascia, including within muscle, or from aponeuroses or tendons. In this location, they may become attached to or encircle underlying bone. Very rarely, they may arise from the subcutaneous tissue. Trauma, chronic scarring, and prior irradiation have been implicated as causative factors. 

Many tumors that were previously classified as fibrosarcomas have recently been reclassified as other fibroblastic tumors. Therefore, prognostic data may not be reliable, and further contemporary studies are needed. After excision, local recurrence rates were reported to approach 60% but were reduced to 25% when postoperative radiation was used. The 5-year survival rate was reported to be 40%-60% in adults.

Metastases often arise within 2 years of diagnosis, but in histologically low-grade tumors, they may take many years to develop. The tumor metastasizes primarily to the lung and bone.

Related topic: infantile fibrosarcoma