Question about thymus

Has anyone ever had PET scan results that came back with "activity" in the thymus area? Mine did, and my doc said not to worry about it, and that we'll just continue with our usual course of action (CT or PET scans every 6 months, and CA125 blood tests every 3 months). He said that there was no "anotomical abnormality" or something like that. I don't know if he's being complacent or what, but I'm nervous. I did have a chest cold and was doing a lot of coughing in the days before the scan---maybe that caused the "activity" (hope, hope!). I did call his nurse and left a message saying, "now please explain to me again why I shouldn't be worried?"