Bone cancer is a malignant tumor that arises from the cells that make up the bones of the body. This is also known as primary bone cancer. Primary bone tumors are tumors that arise in the bone tissue itself, and they may be benign or malignant (bone cancer). Benign (non-cancerous) tumors in the bones are more common than bone cancers. When cancer is detected in bones, it either originated in the bones (as in primary bone cancer) or has spread to the bone after originating elsewhere (a metastasis or secondary cancer that spread to the bones). In fact, when cancer is detected in bone, it most often is a metastasis that has started in another organ or part of the body and then spread to the bones. This cancer that has metastasized to the bone is named for the site where the original cancer began (for example, metastatic prostate cancer that has spread to the bone). Breast, prostate, and lung cancers are among the types of cancers that commonly spread to the bone in their advanced stages. Less commonly, cancer can begin within the bone as primary cancer of the bone, and this is true bone cancer. Primary and metastatic secondary bone cancers are often treated differently and have a different prognosis. There are other cancers that may begin in the bone even though they are not considered to be true bone cancers. Lymphoma is a cancer of the cells that are responsible for the immune response of the body. Lymphoma usually begins in the lymph nodes, but it sometimes begins in the bone marrow. Multiple myeloma is another cancer of the immune cells that typically begins in the bone marrow. These tumors are not considered primary bone cancers because they do not arise from the actual bone cells.