Definition and causes to Muscle tumors – Muscle cancer. A tumor growth in the skeletal muscles is a rare condition, and it will in most cases involve a benign tumor. There are only a few of the malignant tumors (muscle cancer) annually with less than one such case per one million inhabitants.The tumor arises from increased and/or altered growth of muscle cells.Benign tumors are unable to grow into surrounding tissues or the spread with the blood. - But the malignant tumors have this ability.here are only a few of the malignant tumors (muscle cancer) annually with less than one such case per one million inhabitants. The tumor arises from increased and/or altered growth of muscle cells. Benign tumors are unable to grow into surrounding tissues or the spread with the blood.