All cancers start from one particular cell that mutates. Its DNA is damaged by mutations and changes so that the cell no longer grows in an orderly fashion according to the rules for its type. The cancer is often named for the cell type that it grows from but sometimes it has other names.
Soft tissue sarcomas are cancers in which wildly growing (called malignant or cancerous) cells are from a soft tissue part of the body. The soft tissues of the body include fat, blood vessels, nerves, muscles, skin, and cartilage.
Bone is not soft tissue. If the sarcoma grows from bone cells, it is called an osteosarcoma.
You have essentially two kinds of muscle in your body, voluntary and involuntary.
Striated [striped] muscle cells makes up the voluntary muscles; the biceps, triceps, abs, pecs...all the muscles that you can use as you wish by thinking about them. A cancer growing from this kind of cell is a rhabdomyosarcoma.
Leiomyosarcoma is a cancer of smooth muscle cells.
Smooth muscle cells make up the involuntary muscles, which are found in most parts of the body: in uterus, stomach and intestines, walls of all blood vessels, skin [the muscles that give you 'goose bumps']. This muscle is involuntary muscle, you cannot make these muscles move by thinking about them.
GISTs [GastroIntestinal Stromal Tumors] are a special kind of tumor, that used to be considered a kind of Leiomyosarcoma, coming from a particular smooth muscle cell in the stomach/small intestine. We mention these first and separately because there has been a dramatically good advance in treating some of these tumors with a new designer drug. See the separate page on GIST tumors and Glivec/Gleevec. Any GastroIntestinal "sarcoma" or GI LMS should be tested for CD 117 [also called Ckits] protein by an experienced laboratory. This is potentially life-saving information.
Uterine LMS comes from the smooth muscle in the muscle layer of the uterus. Cutaneous LMS comes from the pilo erector muscles in the skin.
GI LMS might come from smooth muscle in the GI tract or, alternatively, also from a blood vessel. At most other primary sites, retroperitoneal [in the abdomen, behind the intestines], extremity, truncal, abdominal organs and so on, the LMS probably grows from the muscle layer of a blood vessel. These blood vessels are everywhere throughout your body, so LMS COULD have a primary site ANYWHERE in your body where there is a blood vessel.