Cancer is a disease of cells—the building blocks of tissue in the body. Inside of cells are coded instructions, called genes, for building new cells and controlling how cells behave. Changes in genes, called mutations, can cause normal cells to become cancer cells. What causes genes in cells to change isn’t fully known.
Over time, cancer cells may form into a mass called the primary tumor. If not treated, the primary tumor can grow through the outer parts of an organ or structure and into other tissue. This is called invasion. Invasion is the second key trait of many cancers.
Third, cancer cells can leave the tissue in which they started and spread to other sites in the body. This process is called metastasis. Cancer cells can spread through blood or lymph. Lymph is a clear fluid that gives cells water and food. It also has white blood cells that fight germs. Cancer cells that have spread can grow and replace many normal cells in the new site.
Scientists have learned a great deal about cancer. As a result, today’s treatments work much better than in the past. Also, many people with cancer have more than one treatment choice.