Soft tissue sarcomas are classified histologically according to the soft tissue cell of origin, though the cell type is not part of the prognostic staging system. Additional studies, including electron microscopy, histochemistry, flow cytometry, cytogenetics, and tissue culture studies may allow identification of particular subtypes within the major histologic categories. The histologic grade reflects the metastatic potential of these tumors more accurately than the classic cellular classification listed below.[1,2,3] Overall, malignant fibrous histiocytoma is the most common histologic type (40% of the total) followed by liposarcoma (25%); however, frequency of histologic type is site-dependent. Pathologists assign grade based on the number of mitoses per high-powered field, presence of necrosis, cellular and nuclear morphology, and the degree of cellularity; discordance among expert pathologists can reach 40% on prospective review.[3,4]

Soft tissue sarcomas include the following tumors:

• Alveolar soft-part sarcoma.[5]
• Angiosarcoma.[6]
• Dermatofibrosarcoma protuberans.[7]
• Epithelioid sarcoma.
• Extraskeletal chondrosarcoma.
• Extraskeletal osteosarcoma.
• Fibrosarcoma.
• Gastrointestinal stromal tumor (GIST).
• Leiomyosarcoma.
• Liposarcoma.
• Malignant fibrous histiocytoma.
• Malignant hemangiopericytoma.
• Malignant mesenchymoma.
• Malignant schwannoma.
• Malignant peripheral nerve sheath tumor.
• Peripheral neuroectodermal tumors.
• Rhabdomyosarcoma.
• Synovial sarcoma.
• Sarcoma, NOS (not otherwise specified).

GISTs are mesenchymal in origin and are immunohistochemically distinct from leiomyosarcomas, schwannomas, and fibrosarcomas with which they are often classified. They can be distinguished by being CD34 positive and CD117 positive. These tumors express a growth factor receptor with tyrosine kinase activity called c-kit (CD117). GISTs of the stomach wall are considered malignant when they exceed 5 to 10 cm, have a high mitotic index, or have metastasized. GISTs of the small bowel are considered malignant if they have any mitoses or are larger than 2 cm. Current evidence suggests that c-kit mutations are more commonly identified in malignant GISTs than in benign GISTs. Malignant GISTs are also usually CD34 positive.[8,9,10,11]

References:

1. Marcus SG, Merino MJ, Glatstein E, et al.: Long-term outcome in 87 patients with low-grade soft-tissue sarcoma. Arch Surg 128 (12): 1336-43, 1993.
2. Soft tissue sarcoma. In: American Joint Committee on Cancer.: AJCC Cancer Staging Manual. 6th ed. New York, NY: Springer, 2002, pp 193-7.
3. Gaynor JJ, Tan CC, Casper ES, et al.: Refinement of clinicopathologic staging for localized soft tissue sarcoma of the extremity: a study of 423 adults. J Clin Oncol 10 (8): 1317-29, 1992.
4. Alvegård TA, Berg NO: Histopathology peer review of high-grade soft tissue sarcoma: the Scandinavian Sarcoma Group experience. J Clin Oncol 7 (12): 1845-51, 1989.
5. van Ruth S, van Coevorden F, Peterse JL, et al.: Alveolar soft part sarcoma. a report of 15 cases. Eur J Cancer 38 (10): 1324-8, 2002.
6. Fury MG, Antonescu CR, Van Zee KJ, et al.: A 14-year retrospective review of angiosarcoma: clinical characteristics, prognostic factors, and treatment outcomes with surgery and chemotherapy. Cancer J 11 (3): 241-7, 2005 May-Jun.
7. Mendenhall WM, Zlotecki RA, Scarborough MT: Dermatofibrosarcoma protuberans. Cancer 101 (11): 2503-8, 2004.
8. Pidhorecky I, Cheney RT, Kraybill WG, et al.: Gastrointestinal stromal tumors: current diagnosis, biologic behavior, and management. Ann Surg Oncol 7 (9): 705-12, 2000.
9. Wang L, Vargas H, French SW: Cellular origin of gastrointestinal stromal tumors: a study of 27 cases. Arch Pathol Lab Med 124 (10): 1471-5, 2000.
10. Nishida T, Hirota S: Biological and clinical review of stromal tumors in the gastrointestinal tract. Histol Histopathol 15 (4): 1293-301, 2000.
11. Miettinen M, Sarlomo-Rikala M, Lasota J: Gastrointestinal stromal tumors: recent advances in understanding of their biology. Hum Pathol 30 (10): 1213-20, 1999.