The most common variety of soft tissue sarcoma is:

MALIGNANT FIBROUS HISTIOCYTOMA   (PLEOMORPHIC SARCOMA) Malignant fibrous histiocytomas (MFH), also called pleomorphic sarcoma now, represent approximately 20-30% of all soft tissue sarcomas. It is the most common soft tissue sarcoma and is the most frequent sarcoma associated with radiotherapy. The tumor occurs more commonly in males and more frequently in the age group of 5th to 7th decades. Most common locations are the lower and upper extremities and retroperitoneum. It begins as a painless, enlarging mass, generally in relation to skeletal muscle, deep fascia or subcutaneous tissue. The tumour is believed to arise from primitive mesenchymal cells, which are capable of differentiating towards both fibroblastic and histiocytic cell lines. Reference:  Textbook of Pathology, Harsh Mohan, 7th ed page no 856