They usually wait 90 days or so after your last treatment for the PET scan. Mine will be scheduled the second week in December. I got a CT scan about 6 weeks after my last treatment as I've been told that even after your last treatment, the radiation/chemo still continues to work for a while. They want to wait until a person gets the full effect of treatment before they do the scan to make sure it has the full amount of time to kill all of the cancer cells.