Our brains are made up of 100 billion cells called neurons, as well as trillions of support cells called glia. Sometimes, during the life cycle of these cells, things can go wrong—changes in your DNA, known as mutations, can cause abnormal cells to grow. When these abnormal cells divide uncontrollably and then gang up together, a tumor forms. A tumor in the brain can be benign (not cancer) or malignant (cancer). In the U.S., about 80,000 new tumors occur each year in the brain or another part of the central nervous system (CNS), such as the spinal cord. “One half of these are cancerous,” says Yale Medicine’s Veronica Chiang, MD, a neurosurgeon and director of the Gamma Knife Center, "and the incidence of brain cancer seems to be on the rise. " The majority of brain cancers are metastatic (meaning they start somewhere else in the body and spread to the brain), but some are primary (they originate in the brain). Though some cases are inherited, it’s unclear what’s behind most primary brain cancers. Evidence suggests that increasing age and exposure to ionizing radiation, used in some cancer treatments, are risk factors. Investigations of other factors, like diet and cell-phone use, have been inconclusive. Treatment can include surgery, radiation therapy and chemotherapy or some combination of these options. At Yale Medicine, an individual treatment plan is developed for each patient with input from multiple specialists who meet weekly to discuss cancer cases.