When cells divide abnormally and uncontrollably, they can form a mass or group of tissue. This growth is called a tumor. Bone tumors form in the bones. As the tumor grows, the abnormal tissue can crowd out healthy tissue. Tumors can be benign or malignant. Benign tumors are not cancerous. Although benign bone tumors usually remain in place and are unlikely to be fatal, they are still abnormal cells that may require treatment. Benign tumors can grow and compress healthy bone tissue, causing future problems. Malignant tumors are cancerous. Malignant bone tumors can cause cancer to spread throughout the body.