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Bowel cancer screening - an alternative strategy

Bowel cancer screening - an alternative strategy

Two recent opinion pieces rightly extolled the merits of screening for bowel cancer. I applaud Dr Parry for leading the introduction of screening this year. Shown to be more cost-effective than lots of other screening tests, it is sad that it has taken so long to eventuate - how many lives have been lost while we waited?

Professor Cox argued that flexible sigmoidoscopy (FS) should be the test New Zealand uses for bowel screening. Dr Parry reaffirmed the Ministry of Health National Bowel Screening Programme’s decision to use faeces testing (FIT) as the screening test. For both tests, a positive result mandates colonoscopy is performed.

FS reduces mortality rates from bowel cancer but is not actually used as the national official screening test anywhere in the world.

FS examines less than half the large bowel and consequently will always miss cancers in the unexamined bowel (the right side). More than 30% of cancers are right-sided, higher in women, the rate increasing recently. Right-sided cancers have a lower survival rate than left-sided. Thus FS only lowers death rates in left-sided cancers.

As an experienced endoscopist, I can say that the procedure is uncomfortable for many and downright painful for some patients, the view obtained is often poor, misses some growths, can cause complications and is used mainly to exclude obvious abnormalities.

A recent review collated data from three trials and concluded that FS screening reduced the incidence of (and death from) bowel cancer in men and women younger than 60 but not significantly in those over 60. There was no effect of screening in the right side in older women.

Professor Parry argues for using FIT, as used in every organised official programme in the world. However there are major differences between what is proposed in New Zealand and what happens elsewhere.

FIT is not just positive or negative. The result is on a continuum and it is decided what cut-off level (of haemoglobin concentration) above which will be deemed positive. If a low level is chosen, the test is more sensitive; more cancers will be picked up but at the cost of more false positives. This means more colonoscopies will be requested and a greater percentage of them will show nothing. If a higher level is chosen as the positive cut-off, then fewer cancers will be picked up but fewer unnecessary colonoscopies will be performed. It depends on resources; if a country cannot supply more than a certain number of colonoscopies, a high level is chosen. An update in the World Journal of Gastroenterology stated: “Values between 20 and 30 g/g are recommended” and this depends on the epidemiology of the population being studied. Remember, we have one of the highest rates of bowel cancer in the world. The New Zealand the cut-off will be 40 ug/g.

The age at which screening begins varies throughout the world. Younger people will also benefit from screening but because they have a lower incidence of cancer, their rate of screening positivity is lower and therefore screening less cost-effective. In most countries screening begins at age 50. In Australia it is or will be, 50. In Japan, 40. In New Zealand, it will be from age 60.

FIT is only partly accurate, with many false positives and some false negative results: If you have
• cancer, the test is correctly positive 70% of the time with a cut-off of greater than 20ug/g. It will however be 90% if the cut –off is 20ug/g. The average is 79%
• a benign polyp (pre-malignant), the test will be positive only 20% of the time
• no cancer, you’ll have a positive test 6% of the time.
• a positive test, you’ll have cancer about 3.4% of the time and a polyp 51.5%.
• a negative test, then you will not have cancer about 93% of the time but the efficient rule-out-cancer levels are only when the cut-off is 20ug/g and
• FIT is less sensitive for right-sided growths.

A Dutch study confirmed these results, using a very low FIT –positive threshold. They performed colonoscopy on over 1000 patients who had provided a FIT test prior to the examination. 20% of the cancers found by colonoscopy had been missed by the FIT test and 72% of the pre-malignant polyps.

By detecting cancers at an early stage, FIT reduces mortality from bowel cancer, but it has not been shown to reduce the incidence of cancer developing in the first place.

There is a third way. I believe that New Zealanders should be given the full information to understand the available data. They should not be under the impression that biennial FIT or 5-yearly sigmoidoscopy are actually the best screening tests available. They might be all that the New Zealand public health system can currently manage but each individual needs to be able to make their own choice.

Colonoscopy is the most accurate test available for bowel cancer screening. It is the “gold standard” according to the World Gastroenterology Association (WGA). It detects cancers, but also searches for polyps, benign growths which eventually become cancers. At colonoscopy, these polyps are removed thus preventing cancer developing. It is the only test that does that. Long-term follow-up in the United States National Polyp Study demonstrated an approximately 90% reduction in the incidence and mortality of colorectal cancer after polyps were removed. This is because virtually all bowel cancers develop from pre-malignant polyps. FIT screening is performed every two years, colonoscopy every 10.

Colonoscopy has complications: 85% are when a polyp is removed rather than just when the bowel is examined. Bowel perforation (0.03%) and bleeding (0.24%). are the most serious. Although hospitalisation is required, most patients can be managed without surgery. Overall complication rate is 0.28%, but reduces with greater endoscopist experience. Improved techniques over the last 15 years have reduced rates.

Colonoscopy is used as a screening test, either organised or opportunistic in USA, Germany, Austria, Poland, Jamaica, Greece, Canada, Brazil, Taiwan, with many other countries such as Norway and Sweden running pilot programmes.

The American College of Gastroenterology and WGA guidelines recommend screening be colonoscopy every 10 years, beginning at age 50.

A Swedish study this year reported that colonoscopy is highly cost-effective. Once only colonoscopy at age 60 was more cost-effective than FIT. If 1000 patients were offered screening, colonoscopy yielded better quality of life for the group and saved 47,200 Euro. Repeating the colonoscopy 10 yearly was even better but was expensive.
In summary, current evidence indicates that colonoscopy is the best screening test, even if performed just once at age 60, but ideally every 10 years from age 50.


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