Colon Cancer

Incidence
Bowel cancer has become the most frequently occurring cancer (other than skin) in Australia. One Australian in 21 will develop bowel cancer, one in 17 males (second most common after prostate at 1 in 11 males) and one in 26 females (second most common after breast at 1 in 11 women),

Since 1975, there has been a rapid increase in the incidence of bowel cancer in Australia which appears to be related to Western lifestyle. Australia, together with New Zealand, the United Kingdom and the United States of America has one of the highest incidences of bowel cancer in the world. Migrants to Australia from low risk countries begin to show an increased risk of bowel cancer within ten years of arrival, and migrant communities will have reached the general Australian risk within two generations.

Bowel cancer is seen most commonly in people between the ages of 50 and 80, but it can occur in teenagers, and is seen in people in their twenties and not uncommonly in their thirties.

Although some are at higher risk, (in particular, those with a strong family history of bowel cancer), 85% of patients presenting with bowel cancer will have no known risk factors.

Surgical cure rates
There have been significant advances in the treatment of established bowel cancer in the last 30 years. These gains, somewhat marginal, have been the result of using newer techniques to avoid the necessity for permanent stomas and by the development of specialised training programmes in colorectal surgery, used in combination with sophisticated radiotherapy and chemotherapy treatment schedules.

Overall we cure 50% of bowel cancers that present with symptoms, but that proportion is higher in patients whose cancers are diagnosed as part of a screening program and at an earlier stage. The improving rate of cure for bowel cancer in NSW, now up to 60%, thus seems to be due to earlier diagnosis. In turn, earlier diagnosis seems likely to be due in part to greater community awareness of the need to investigate bowel symptoms.

Screening for polyps
In addition to early diagnosis of bowel cancer, screening programs can prevent the disease. Nearly all bowel cancers start as polyps, small “warty” growths which appear on the surface of the bowel lining. Polyps are very common, especially as people become older, most developing during and beyond the fifth decade of life. Most polyps do not become cancerous; but (almost) all cancers arise from a polyp. Note that, with breast and prostate cancer screening, a positive test indicates cancer (albeit early in development and thus potentially curable), whereas the identification and removal of colonic polyps will actually prevent cancer. Indeed it takes probably between 7 and 14 years for a polyp to become cancer.

The Colorectal Foundation

Types of screening
Broadly speaking, there are three forms of screening available at present. Testing for unsuspected blood loss in the bowel, called faecal occult blood testing (FOBT), radiological techniques, such as barium enema and computerised tomographic colonography (or “virtual” colonoscopy) or colonoscopy.

Faecal occult blood testing has been shown in large overseas studies to decrease the overall deaths from bowel cancer by 15%. Its main purpose is to diagnose bowel cancer early, and is poor in the diagnosis of polyps, and as such is of little value in prevention of bowel cancer. The test is easy, without risk, and in itself relatively cheap, but as the likelihood of having a bowel cancer with a positive test is much higher than the normal population, a positive test effectively commits the patient to a colonoscopy with its attendant costs and risks, and at least 50% of those colonoscopies are normal. It has the potential to miss 12% of cancers.

Radiological techniques will miss up to 30% of significant polyps, and will fail to diagnose up to 5-10% of bowel cancers. In the event that a polyp is shown on one of these techniques, colonoscopy is then necessary to confirm the diagnosis and to remove the polyp. While the technique of the “virtual colonoscopy” continues to develop, the current position, as expressed in recent editorials in the Lancet and the Medical Journal of Australia that the software capabilities are not yet sufficiently developed to allow widespread use. Again, a positive finding will require a subsequent colonoscopy for confirmation and therapy.

Colonoscopy will define 95-100% of significant polyps, and nearly all cancers, thus remaining the most widely used form of screening, both because of its accuracy, and because it is possible to remove the vast majority of polyps at the time of diagnosis. Drawbacks are the cost to the community and inconvenience to the patient, as well as the important possibility of complications. Although as many as one colonoscopy in every thousand in Australia is reported to result in a bowel perforation, experience and skill will reduce the risks to as low as one in 10,000.

Future Issues
Among the issues related to screening which require requiring clarification are: age of commencement of screening, appropriate screening intervals, cost-effectiveness of various modalities in the Australian setting, the most effective methods of combining the screening modalities, and a clear definition of the utility of various tests in people of differing risk profiles.

These issues require definition, not only because of the obvious benefits to the community from effective prevention and early diagnosis of bowel cancer, but also because of the overall cost of such screening to the community. For example, lifetime screening applied now for all Australians between the ages of 50 and 69 years, using the cheapest test (FOBT) carried out every two years, together with follow-up colonoscopy of all those with positive results, has been estimated to cost more than $2 billion annually. It is clearly important that efforts be made to answer the questions posed above and epidemiological research be undertaken to answer these questions should be of high priority.