When a malignancy arises from the connective tissues deep to a surface it is know and a sarcoma. Most cancers arise from cells on the surface of a structure and are known as carcinomas.
Soft tissue sarcomas can develop from soft tissues like fat, muscle, nerves, fibrous tissues, blood vessels, or other deep skin tissues. They can be found in any part of the body.
Most of them develop in the arms or legs. They can also be found in the trunk, head and neck area, internal organs, and the area in back of the abdominal cavity known as the retroperitoneum.
There are about 50 different types of soft tissue sarcomas, including Liposarcoma, Leiomyosarcoma, Synovial Sarcoma, Angiosarcoma, Fibrosarcoma, Mailgnant Fibrous Histiocytoma, Gastrointestinal Stromal Tumor, and others.
The mainstay of treatment is surgery. Sometimes radiation therapy and chemotherapy are used. Targeted therapy is also used for certain types.
For more information look here: http://www.cancer.org/cancer/sarcoma-adultsofttissuecancer/detailedguide/sarcoma-adult-soft-tissue-cancer-soft-tissue-sarcoma or here: http://sarcomaalliance.org/what-you-need-to-know/sarcoma-specific-information