Bone cancer may be classified as "primary tumors" which originate in the bone, and "secondary tumors" which originate elsewhere. Primary tumors come from bone cells which are growing out of control, and are called "sarcomas". The bone marrow makes blood cells, and multiple myeloma, which is a form of cancer of certain blood-forming cells, also frequently presents as one or more bone tumors. Secondary bone tumors include metastatic tumors which have spread from other organs, such as the breast, lung, and prostate. Metastatic tumors more frequently involve spine bones than others. Tumors which originate in the soft tissues near bones may also secondarily involve bones through direct invasion