A tumor refers to an abnormal growth in the body. Tumors are made up of abnormal cells that are uncontrollably dividing and replicating themselves. Tumors can be classified as benign or malignant (cancerous). Depending upon the type of cell that the tumor is derived from, a malignant tumor can be classified as a sarcoma or a carcinoma. Malignant tumors can also be considered primary or secondary. In terms of bone tumors, a primary bone tumor arises directly from a particular bone. Secondary bone tumors are tumors that involve the bone but have traveled from a cancer in another part of the body. Primary malignant bone tumors are sarcomas. Secondary bone tumors are called metastatic bone tumors or metastatic bone cancers and most are carcinomas that have traveled from other primary tumors such as breast cancer, lung cancer, prostate cancer, kidney cancer, thyroid cancer and gastrointestinal cancer. For instance, breast cancer that spreads to the bone; the breast cancer is the primary cancer and the breast cancer that is in the bone is a secondary cancer or a metastatic cancer.