Diagnosis of colon and rectal cancer

A digital examination of the rectum is part of the routine examination of the rectum by every doctor and should be able to palpate the majority of rectal cancers.

 

A faecal occult blood test should alert the doctor to a bleeding lesion (not necessarily a tumour) in the bowel.

 

However, the most important test is the proctoscopy and colonoscopy where the doctor directly visualises the entire rectum and colon and is able to biopsy (take for analysis) any parts of the lining wall (the site of origin of all cancers) that look suspicious for cancer.

 

An indirect alternative to examine the large bowel is the barium enema where the colon is filled with radio-opaque barium solution such that a negative image is projected on x-ray film and any irregularities of the colonic wall are noted.

 

The biopsy for microscopic (histological) analysis is taken at the time of the colonoscopy and once the diagnosis is established, then a CT scan of the abdomen is performed to determine if the disease has spread outside the colon/rectum. A chest x-ray and blood tests (including the serum marker proteins that may be elevated in this disease viz. CEA (carcino-embryonic antigen) and CA-19-9 are also performed at this time.

 

CEA is a cell surface glycoprotein that is readily expressed on most colorectal epithelial (lining) cells; indeed, it was originally though to be exclusive to the large bowel. CEA is shed into the blood stream where its serum concentration is measurable; it is cleared from the blood by liver metabolism (thus liver dysfunction is a cause for a raised level in the blood). Although it is now known that there are many other cancers that will cause a rise in blood CEA and other illnesses too, nevertheless, this serum test is easy to perform and when a high level is recorded (say five times the upper limit of normal or more) then malignancy, particularly of the large bowel should be suspected.

 

In the follow-up of treated large bowel cancer patients, the use of serial serum levels can be useful; a rising level suggests relapse.

 

CA 19-9 is another cell surface antigen, this time an oligosaccharide (related to a blood group antigen), and this is expressed particularly in gastrointestinal cells and cancers derived from these cells.

 

Serum levels of CA 19-9 more than 100 U/ml are rarely found in benign disease (although they may occur in some forms of obstructive jaundice) and can be used to diagnose and monitor disease activity in colorectal cancer in a similar way that has just been described for CEA.