Cancers of the colon and rectum are the commonest forms of non-skin cancer in the Republic. They account for 10 per cent of all cancers diagnosed, with almost 2,000 people affected each year.
Unfortunately, about half of these patients will have died within five years of diagnosis, which suggests they are presenting late with the disease. Is there any way we can improve the pick-up rate for colorectal cancer?
According to Dr Padraig McMathuna, consultant gastroenterologist at the Mater Hospital in Dublin, a family history of the disease is a key factor in deciding who should be screened. Both the proximity and the number of relatives with a history of colon cancer are determining considerations. For example, if you have a first-degree relative (mother, father or siblings) who has developed bowel cancer below the age of 55, you should present early for screening. The programme for screening ideally starts when you are 10 years younger than the age at which your close relative was diagnosed. So, if they were 55, you should start screening at 45.
There are no hard and fast rules about colon cancer screening and each person's risk must be calculated individually. Indeed, Dr McMathuna believes that one of the challenges in this field is to develop guidelines for bowel cancer screening which are easily understood by the public.
Why not screen the entire population? The main reason is that the only reliable screening procedure at present is a colonoscopy, which involves passing a flexible tube up through the length of the colon. A colonoscopy is completed in a day, using sedation, but patients must first clear out their bowels using a special preparation, a process which takes several days. Because this kind of screening is a specialised and expensive procedure, it is reserved for those at increased risk.
A test called faecal occult blood, in which faeces from a rectal examination are tested for microscopic traces of blood, has been found to be unreliable. There is a trial currently underway in the UK, using once-off flexible sigmoidoscopy between the ages of 50 and 65 as a screening test. This would examine the left side of the bowel only (where most tumours occur) and would require less expertise; it will be interesting to see if it can reduce the mortality from bowel cancer. Unfortunately, there is no blood test available for the disease.
Another challenge posed by colorectal cancer is the fact that the length of time a person has suffered symptoms does not correlate with the extent of the spread of the disease at diagnosis (see panel). Unlike the case of skin cancer, in which visible changes usually correlate with the extent of the disease, it is possible for a person with a single episode of rectal bleeding to have a tumour which has already invaded the bowel wall.
WHICH brings the emphasis back to finding a good method of targeted screening. Dr Helen Fenelon, a consultant radiologist at the Mater Hospital, has recently returned from Boston, where she was involved in developing an exciting new technique called virtual colonoscopy. Most bowel cancers start as polyps - small finger-like projections - in the bowel and the Boston research found that by using a multi-splice CT scanner, it was possible to pick up 90 per cent of polyps greater than one centimetre in size. This compares well with the diagnostic rate of colonoscopy itself.
A computer puts all the images together to create a three-dimensional picture of the inside of the colon. It is particularly good at visualising the right side of the colon, which can be a problem with a percentage of colonoscopies.
Dr Fenelon has joined with Dr McMathuna and researchers from Dublin City University in an Irish Cancer Society-sponsored study of virtual colonoscopy. If, as expected, the two-year research programme proves the effectiveness of the new technique, it is likely to replace real colonoscopy as the screening test of first choice within five years.
Because it takes 10 minutes (compared to a possible 45 minutes for a real colonoscopy) and there is no sedation involved, it will be possible for a person to return to work immediately after a virtual colonoscopy. So there will be significant cost benefits associated with the new procedure.
The entire issue of preventing colon cancer in the first place is something this column will return to in the near future. With research emphasising the beneficial effects of aspirin and throwing into question those of a high-fibre diet, there is considerable scientific activity in this area at present.
Dr Muiris Houston, Medical Correspondent, can be contacted at mhouston@irish-times.ie or messages can be left on tel: 01-6707711, ext 8511. Dr Houston regrets he cannot reply to individual medical problems.