I had a 3 post treatment PET/CT scans and then it was stopped. Insurance was just one of the reason so we switched to an MRI (this year).
Regarding radiation this is a concern and there are numbers floating around that correlate medically unnecessary CT scans (in ERs), done to avoid lawsuits and induced cancers. While these numbers are not zero they are very small and in comparison to the radiation which you already got it is a drop in the bucket. (This is my rationale).
Still, perhaps you could explore if MRI's are an option for you.

M