PET scan will certainly be able to differentiate between scar tissue and cancer.
But the timing of PET scan is very very important. After radiation
therapy, it is ideal to wait for 12 weeks before radiation induced inflammation becomes normal. If FDG uptake occurs after 12 weeks from completion of radiation, it is not radiation induced inflammation.
At 7 weeks, it become difficult to conclusively say if its cancer or radiation induced inflammation. But the dictum if radiation induced FDG uptake is usually diffuse and low to moderate grade. A focal area of uptake usually indicates another problem like cancer or infection.
Recurrent cancer usually is focal and intense, though its not a dictum in all the cases.