Can an abdominal pelvic cat scan screen for colon cancer or see a mass in the rectum?

Sometimes. Sometimes a cat scan will be able to see a mass in the colon or rectum but it is not a good test for screening for colorectal cancer. There is a specialized type of ct scan that creates a virtual image of the inside of the colon that is sometimes used when a patient is not a good candidate for a colonoscopy.
Potentially. If the masses are large enough, they can be seen on cat scan. In order to increase the chances of these being seen, the cat scan has to be administered as a "ct colonography". Oral contrast agent needs to be swallowed, and an air enema needs to be administered during the ct scan. Traditionally, colonoscopy is considered the most reliable and accessible way to screen for colorectal cancer.
Possibly & Yes. These are 2 different scenarios. Screening is performed in asymptomatic patients. You can screen for colon cancer with a special type of ct scan called virtual colonscopy. If a polyp is found it would still need to be removed with a colonscopy masses in the rectum can be seen with a pelvic ct scan.