Gut cancers commonly occur either in the top end (stomach and oesophagus) or the bottom end (colon and rectum), with cancers of the middle bit (the small bowel) being comparatively rare. There are some interesting trends in the patterns of gut cancers which I will run through, starting at the top with stomach cancer.
Overall, almost a million people are diagnosed with stomach cancer each year with around two-thirds of those afflicted dying from the disease – at least 650,000. Stomach cancer has been steadily falling in incidence in the West over the last 50 years, as illustrated in Figure 1, moving from being a relatively common cancer to now being quite rare. In other parts of the world, incidence has also begun to fall but more recently. Various reasons have been proposed for this, ranging from the rise of cheap refrigerators to medical treatments for stomach ulceration, but at present, the reasons for the changes are not fully understood.
Cancers of the large bowel also show large variations between populations. Broadly speaking, bowel cancer is common in Europe and North America, less common in the Far East and uncommon in Africa. It is thus predominantly a disease of the developed world. Altogether, around a million people are diagnosed with the disease each year, and around half of these patients will die from the disease. Death rates are now declining in North America and Europe due to improved awareness, early diagnosis, and better treatment. Studies of migrants suggest that the differences are environmental rather than racial – migrants from low-risk to high-risk countries rapidly take on the risk pattern of their new homeland. In addition, countries with an increasingly westernized diet such as Japan are seeing a rise in the incidence of the disease. The prime candidate for this effect is, therefore, diet – differences in the environment of the lining of the lower bowel clearly arise from differences in what goes in at the top end! There thus appears to be some sort of reciprocal effect – changes in diet over the last 50 years have made stomach cancer increasingly rare but have led to an increased risk of cancer at the other end of the bowel. Studying these sorts of changes provides important clues to the origins of cancers and also can point the way to prevention strategies.