A new warning this afternoon about dental X-rays.  A study published in the Journal "Cancer" shows a link between frequent X-rays and developing a certain brain tumor called Meningioma.   Researchers found patients who had bite wing X-rays at least once a year had a 40-to-90 percent chance of getting the tumor. They say this doesn't mean you should stop going to the dentist, nor does it prove X-rays cause tumors. They  say annual X-rays put you at greater risk. American dental association guidelines say healthy adults without cavities should get a dental X-ray once every two or three years.  Meningiomas are the most frequently diagnosed brain tumor among adults.