Hello. My dad has had PET/CT scans throughout his cancers. First, he had it prior to treatment to see what was going on in there, if there were any areas of increased metabolic activity where there could be cancer. Then he had it periodically during treatment to see if treatment was producing any effects on the cancer.
After you go for a scan, your doctor receives a "radiology report," that describes what they saw. I think it's good to get a copy of these reports as you go along.