Usually the muscle cancer is developed in children and teens while the cells are still dividing. Once a person reaches adult stage the probability to develop it is very small. Once a person is adult the muscular mass is developing more actin and myosin inside the the cell during the repair phase. There are some cases in which a muscle cell might still divide in adulthood (when the density of actin and myosin is big and especially in the case of the steroid use) but generally this is not happening. This might be one of the factors that reduces the possibility of indiscriminate and continuous multiplying found in cancer cell.

As Vlad mentioned, terminally differentiated cells such as muscle and neurons do not typically become cancerous. The cells that are usually cancerous are the ones that exhibit a high rate of cell division, and usually pick up a mutation during this process. For example, glioblastomas in the brain are very vicious cancerous cells that are glial cells of the nervous system (typically astrocytes) and not neurons.

The development of a "cancer" correlates well with the frequency of replication of cell type. Epithelial cells replicate whereas muscle cells and neurological cells replicate far less frequently and sometimes not at all. However, during development of the fetus and during the growth phase of the young, the chance for mutations is present and at this time it is possible for the appearance of a "cancer cell: that escapes detection, etc., and in later years gives rise to a full-blown tumor which may be malignant "cancer"). Of course these events are far less frequent than those that take place at a much later time in life.

I am sorry sir, if the question is not clear to you. I meant most of muscle cancer are benign in nature, not malignant. But it should be other way around, since they are richly supplied with blood vessels, the cancerous cells can become malignant and spread through circulatory system.