Pretty good description. A PET scan measures metabolic activity, and will show any areas that are actively eating up the radioactive sugar at an excelerated rate. Tumors do that. CT scans look at structures. They just show what masses are where. The radiologist has to figure out what all the images mean. The contrasts help differentiate some stuff from what's around it, but it can't tell what's malignant and what's not. My oncologist likes CT scans with oral, IV and rectal contrast because you get clearer pictures of potential masses. You can figure out what they are later. PET scans are good for initial staging to see if and where there is any spread, particularly to lymph nodes. Later, I think, CTs are good for looking for masses that might not have been there before.