The soft-tissue sarcomas are a group of diseases characterized by neoplastic proliferation of tissue of mesenchymal origin. Thus, they differ from the more common carcinomas, which arise from epithelial tissue. Sarcomas can arise in any area of the body and from any origin; however, they most commonly arise in the soft tissue of the extremities, trunk, retroperitoneum, or head and neck area. There are more than 50 different types of sarcomas, classified according to lines of differentiation toward normal tissue. For example, rhabdomyosarcoma shows evidence of skeletal muscle fibers with cross-striations, liposarcoma shows fat production, and angiosarcoma shows vessel formation, and there are several types within each of these groups. Precise characterization of the types of sarcoma is often impossible, and these tumors are called unclassified sarcomas. All of the primary bone sarcomas may arise in soft tissue, leading to such diagnoses as extraskeletal osteosarcoma, extraskeletal Ewing sarcoma, and extraskeletal chondrosarcoma. A common diagnosis in the recent past was malignant fibrous histiocytoma (MFH). This tumor is characterized by a mixture of spindle (or fibrous) cells and round (or histiocytic) cells arranged in a storiform pattern with frequent areas of pleomorphic appearance and frequent giant cells. There is no evidence of differentiation toward any particular tissue type. Many tumors previously called pleomorphic fibrosarcoma, pleomorphic rhabdomyosarcoma, and so forth were classified as MFH. As immunohistochemistry and molecular diagnostic techniques have improved, many of the tumors previously classified as MFH have been reclassified
as pleomorphic something else. Furthermore, there are strong opponents of the term MFH because there is no evidence that the tumors have either fibrous or histiocytic origin, and pleomorphic tumors previously classified as MFH are frequently now referred to as unclassified high-grade pleomorphic sarcomas.

Metastatic spread of all sarcomas tends to be through the blood rather than through the lymphatic system. The lungs are by far the most frequent site of metastatic disease. Local sites of metastasis by direct invasion are the second most common area of involvement, followed by bone and liver. (Liver metastases are common with intra-abdominal sarcomas, especially gastrointestinal stromal tumors [GISTs]; however, metastases to soft tissue are common with myxoid liposarcomas.) Central nervous system (CNS) metastases are extraordinarily rare except in alveolar soft-part sarcoma.

Staging of sarcomas is complex and demands an expert sarcoma pathologist. Tumors have been staged according to two systems: the American Joint Committee on Cancer (AJCC) staging system and the Musculoskeletal Tumor Society staging system. The new International Union Against Cancer (UICC)/AJCC staging system, with international acceptance, takes portions from each of the older systems and more appropriately identifies patients at increased risk of metastatic disease. Further revisions to this system are still under way, and a final, widely accepted system is still not universally accepted. As current and older publications still refer to the older systems, however, all will be included.

Patients are evaluated and followed according to the plan in Table 16.1.