Colon cancer is one of the more common cancers. CT is not a good test to pickup colon cancer. Colonoscopy is the standard for screening and diagnosis. Since CT is done so commonly for many reasons, at times, it picks up suspicious abnormalities which turn out to be colon cancer. So most of the time, colon cancer is discovered as an unexpected finding rather then as the reason for the test.
On CT, colon cancer is most commonly seen as a mass or a segment of thickened narrowed colon. Unfortunately, some normal findings can look like colon cancer. Stool can be round and mass like and at times confused for cancer. Also, the colon simply doing its job and contracting can look like a narrowed and thickened segment. But at the least, a question of an abnormality should be further investigated, preferably with colonoscopy.
In some cases, I have seen patients present to the emergency room with a colon obstruction from a tumor. Often you see a dilated colon to the point of the mass or tumor. Other reliable signs include lymph nodes next to the tumor mass or spread to other parts of the body. In all cases, the diagnosis will have to be confirmed by colonoscopy and biopsy.
More commonly, I see patients having their tumor staged or followed up for response to treatment. Staging of the tumor usually happens when it’s first diagnosed. The big question is whether the tumor is confined to the colon or has spread elsewhere. Spread of the tumor means that more systemic therapies will be needed. The prognosis is also worse for these tumors.
Patients who are undergoing chemotherapy for tumors which have spread will often have CT scans to see how they are responding and if the therapy is working. Patients in remission will be monitored for any tumor recurrence or spread. These patients get serial CT scans at an interval determined by the oncologist.
CT is therefore not the best test to diagnose colon cancer. A colonoscopy is the best test followed by biopsy and diagnosis. But CT is sometimes able to detect suspicious abnormalities which need to be looked at with colonoscopy or further imaging tests.
Colon cancer is more commonly staged with CT. Colon cancer treatment is monitored with CT. Therefore, an abnormality identified on CT does not mean you have colon cancer. But it does mean that you need further testing to be sure.