@ina3 – good morning. I'm happy to hear that the PET scan shows that your dad's cancer is not progressing. I'm sorry that your dad has this additional problem. Have his doctors stopped the radiation? What kind of radiation is he having? There are many kinds. Have you asked his radiologist? Memory loss is quite common with cancer treatments. The solution might be that your dad might have to choose between these side effects and continuing with the radiation. What has his doctor recommended?