What tests can be done to detect and diagnose colon cancer?
When colon cancer is suspected, a colonoscopy is typically performed to confirm the diagnosis and locate the tumor.
Colonoscopy is a procedure whereby a health care professional inserts a long, flexible viewing tube into the rectum for the purpose of inspecting the inside of the entire colon. Colonoscopy is generally considered more accurate than barium enema X-rays, especially in detecting small polyps. If colon polyps are found, they usually are removed through the colonoscope and sent to the pathologist. The pathologist examines the polyps under the microscope to check for cancer. Colonoscopy is the best procedure to use when cancer of the colon is suspected. While the majority of the polyps removed through colonoscopes are benign, many are precancerous. Removal of precancerous polyps prevents the future development of colon cancer from these polyps.
Sigmoidoscopy is a procedure performed using a shorter flexible scope to examine just the left colon and rectum. It is more easily prepared for and performed than a complete colonoscopy but has obvious limitations in terms of not being long enough to assess both the right and transverse colons. Polyp removal and cancer biopsy can be performed through the sigmoidoscope.
If cancerous growths are found during colonoscopy, small tissue samples (biopsies) can be obtained and examined under the microscope to determine if the polyp is cancerous. If colon cancer is confirmed by a biopsy, staging examinations are performed to determine whether the cancer has already spread to other organs. Since colorectal cancer tends to spread to the lungs and the liver, staging tests usually include CT scans of the lungs, liver, and abdomen. Positron emission tomography (PET) scans, a newer test which looks for the increased metabolic activity that is common in cancerous tissue, also are employed frequently to look for the spread of colon cancer to lymph nodes or other organs.
Sometimes, the health care professional may obtain a "tumor marker" blood test called a carcinoembryonic antigen (CEA) if there is a suspicion of cancer. CEA is a substance produced by some colon and rectal cancer cells as well as by some other types of cancers. It is sometimes found in high levels in patients with colorectal cancer, especially when the disease has spread. It can serve as a useful test to follow if it is found to be elevated before the cancer is removed. However, not all patients with colorectal cancer will have an elevated CEA even if their cancer has spread. (Some colorectal cancers don't produce it.) Additionally, some patients without cancer can have an elevated CEA blood test. About 15% of smokers, for example, will have an elevated CEA without colon cancer. So the CEA is not used to diagnose colorectal cancer but rather to follow the effects of treatment of colorectal cancer in someone with a known history of the disease because, again, in some patients the amount of cancerous tissue correlates with the level of CEA.