1) A CT scan cannot confirm colon cancer, only tell if there are signs that point to it. For example, I had a CT scan for what was thought to be diverticulitis. What caused concern to the radiologist and my primary was significant sigmoid colon thickening which can be an indicator of cancer. A colonoscopy revealed several polyps and diverticulum which probably caused the thickening on imagery.
So if the CT scan shows normal colon (or the "colon is unremarkable" in their technical jargon). We can pretty much rule out colon cancer, right?