<s>[INST] Hi I was wondering, with a PET scan, should you be able to tell the difference between active scar tissue, and scar tissue from radiation treatment (which ended approx.7 weeks ago)? Also if a PET scan shows inflammation in the lymph nodes, but the lymph nodes appear normal how likely might it be that the inflammation is residual effects from the radiation,or rather that the cancer returned? [/INST] Hi, 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 becomes 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 is usually focal and intense, though it's not a dictum in all the cases. </s>