Can a ct with contrast of abdomen and pelvis detect colon cancer?

Depends: If you mean a regular ct with contrast (and not a virtual colonoscopy), the answer is occasionally. If the cancer is spread, you may see enlarged nodes or metastases. If not spread, you could conceivably see obstruction or changes in the bowel wall but it would likely be a big tumor nonetheless. A ct is not a screening test and is not the way to diagnose early colon cancer.

It can sometimes: Unfortunately, ct scans are not very good at finding small polyps like a colonoscopy does. The scan will usually only pick up larger lesions. The endoscopy has an additional benefit of allowing small polyps to be biopsied or removed. Polyps tend to grow to become cancer, so they should be removed if possible. Ct scan, won't allow treatment at same time, and thus, is not as good a test for colon ca.