Researchers from Brigham and Women's Hospital, Yale University School of Medicine, Duke University, UCSF and Baylor College of Medicine have found a correlation between frequent dental x-rays, which are the most common source of exposure to ionizing radiation in the US, and an increased risk of developing meningioma, the most common primary brain tumor in the US, which accounts for about 33 percent of all primary brain tumors. Researchers studied data from 1,433 patients diagnosed with meningioma between 20 and 79 years of age from May 2006 to April 2011 and compared the information to a control group of 1,350 participants with similar characteristics. They found that patients with meningioma were twice as likely to report having a specific type of dental x-ray called a bitewing exam, and that those who reported having them yearly or more frequently were 1.4 to 1.9 times as likely to develop a meningioma when compared to the control group.