I wish I knew more about this field, but I do know that they are still in demand, especially at major cancer centers. I can't see how we would function without radiation treatment, so I would bet on job security for medical dosimetrists for years to come.
I don't think you'd be exposed to as much radiation as an X-ray tech, because as I understand it, your job would be more behind the scenes, calculating radiation dosages and formulating treatment plans.
Check out this website for the American Association of Medical Dosimetrists (AAMD):
www.medicaldosimetry.org .