Can A CT Detect Colon Cancer?

The quick answer is yes it can, but it’s not reliable.  The best test to detect colon cancer is a colonoscopy done by a gastroenterology doctor.   Given that CT is done so frequently, it is not uncommon to detect a mass or suspicious abnormality of the colon.  In all cases, any suspicious finding on CT needs to be confirmed with a colonoscopy and possible biopsy.

CT scan of the abdomen and pelvis includes the colon in its entirety.  The colon is better evaluated when the CT is done with contrast.  Radiologists will examine the colon for abnormalities on all scans.  A colon cancer can sometimes be seen as a segment of thickening of the wall of the colon.   Sometimes a mass will be seen involving the colon.

A colon cancer can present initially with spread to lymph nodes or the liver.  In these cases, it may not be known from the CT that the cancer is colon.  Other cancers can have this appearance as well.  It is only after a colonoscopy that the cancer will be found.   Sometimes the abnormal lymph nodes will cluster next to the colon which will give a clue.  Other times, the cancer can be seen.

A colon cancer can cause obstruction.  The cancer becomes big enough where the lumen can no longer allow passage of stool.  Often these patients will present to the emergency room.  A CT scan will show a big dilated colon to the level of the colon cancer.  The colon past this point will be small in size and collapsed.   The colon cancer may or may not be seen.  A colon obstruction in an adult will often be from a mass.

Unfortunately, a normal colon can also have a thickened appearing wall when it is collapsed on itself or the lumen is not full of stool.  A normally functioning colon will push stool towards the rectum.  This is a wavelike contraction of the muscle.  This can also lead to a thickened narrowed appearance of the colon.

Other conditions such as diverticulitis or colitis (inflamed colon) can also cause thickening of the colon which can mimic a colon cancer.  In these cases, a colonoscopy may be needed to be sure there is no colon cancer.  Thickening and narrowing can also be caused by inflammation or scarring of colon from inflammatory bowel disease like Crohn’s disease.

More commonly in my experience, we don’t see colon cancer on CT even when it is present.  That is because the tumor may be too small.  There is also stool within the colon which may obscure it.  The radiologist reading the scan may interpret an area of thickening as normal and related to physiology.  There may be multiple areas of colon wall thickening and the radiologist can not raise suspicion for a cancer every time.

Colon cancer on CT is sometimes discovered.  The radiologist will raise suspicion and a colonoscopy will be needed to confirm.  Many colon cancers are not identified on CT.  A colonoscopy is needed for screening and diagnosis.  Occasionally patients will present with a colon obstruction or spread of the cancer.  There are established guidelines for colon cancer screening.