Today, nearly 700,000 people in the U.S. are living with a brain tumor, and yet, when it comes to pinpointing causes or risk factors, scientists are still searching for answers. "Unlike the strong correlation between smoking and lung cancer, we just haven't found a specific risk factor like that for brain tumors," said a researcher. "We have determined that ionizing radiation to the head is a risk factor when received in therapeutic doses, but even in those cases, the risk of developing a brain tumor is low."