Some doctors say, a PET scan is 90% accurate by 8 weeks. Some do them 6-8 weeks, in case further surgery or treatmnt is needed, which has better outcome, and reduces cancer regeneration. Under 6 weeks, a PET was not found to have acceptable findings, 4 months is better than 3 months, and a year post treatment, the PET was found to be 100% accurate. I had about a dozen PET scans, the earliest were mostly 3 months, and one at 5, and another at 8 months. Sometimes they do another scan, like MRI, to get a better look at areas that may have been inconclusive of show better.