CT scans show the entire colon and can occasionally detect abnormalities. Unfortunately, CT is not the best way to evaluate the colon, especially abnormalities inside it like masses. Colonoscopy is the best for that.
Sometimes the radiologist will suggest that a segment or a part of the colon is narrowed or thickened. Unfortunately, this appearance can also be from normal physiology or a normal appearance as well. Therefore, a segment of colon that looks abnormal on CT may need to be further evaluated with colonoscopy before anything is concluded.
Additionally, CT scans are often ordered without oral contrast. Oral contrast is given to drink before the Ct scan. Oral contrast highlights the bowel and makes it easier and better to evaluate. The colon normally works forming and pushing stool and can look narrowed from spasms or from it simply doing it’s job. Other parts of the colon may be narrowed from a lack of stool distending it.
When the colon is not distended, it may look thickened with the walls of the tube touching and folds thickened. Additionally even stool can look like cancer. Sometimes stool can look round and mass like or cause an area of thickening on one side of the colon wall.
There are however some clues which make the narrowed segment more likely to be truly abnormal. If there is a true round mass in the region, lymph nodes adjacent to it, the colon upstream is dilated or there is evidence of tumor spread like a mass in the liver. Other concerning features would be if the patient has anemia, blood in the stool, or weight loss. A family history of colon cancer or other predisposing conditions would also add more concern.
A colon mass or narrowing may obstruct the colon to some degree. This will cause the colon and small bowel to be dilated upstream to the narrowing. Additionally, when there is true mass, often I see lymph nodes next to it or “dirty” fat indicating possible spread to lymph nodes. In more advanced cases, there may be liver masses or masses elsewhere in the abdomen. This will increase the probability that the colon abnormality is indeed real. It will still have to be further evaluated with colonoscopy to be sure.
I have seen some very concerning appearances on CT turn out to be nothing. I have also seen some not so concerning abnormalities turn out to be cancer. Therefore, CT is not very good for diagnosing colon tumors except in advanced cases in my experience. If the CT scan does not pick up a colon tumor, that is more often expected. A colonoscopy is needed for best evaluation for colon cancer. Certainly don’t ignore an abnormality of the colon indicated on CT, but wait until further testing is done before thinking the worst.