Please get it checked ASAP. Bob too had some vison problems and lightheadedness about seven/eight months after treatment. His Rad doc thought it might be from a hardening of the arteries in the radiation field. Turns out it was from diabetes and slightly high blood pressure. They think the diabetes is from his rapid wieght loss and then wieght gain. I had to threaten him to get him to check it out but now with meds and a change in diet he is right as rain...don't mess around with these symptoms any of the causes untreated could be serious but most likely can be treated.