Sara , I would not worry for a few reasons .If you are over 60 ,and did not many Ct s before,13 is not much.The scanners used today use less than half radiation than 10 years Ago.
We are left with not much of a choice because the information is necessary .You should share your concern with your doctor .They may increase the interval of scanning , or screen with a liquid bx ,which is blood drawn and tested for tumor dna,They can use mri without pet and reserve pet for any changes
Best regards
Frank

I would not worry to much about the scans they are relevantly safe today

Sara b Good luck

Thank you both. Being in this clinical trial I get scans more often than normally would be done.