GEOGRAPHIC OF CANCER: INTERCONTINENTAL VARIATIONS
The list of contrasts that can be drawn when the variation in national incidence of cancers is examined across the whole world is endless. We have looked in detail at Europe and in this section we will give a few examples to illustrate that intercontinental differences can be even more dramatic than international differences.
The International Agency for Research on Cancer regularly publishes a compendium of cancer incidence in five continents, the most recent edition of which was in 1987. From these published figures it is possible to pull out some striking examples between the highest incidence and the lowest incidence across the world. The most dramatic is malignant melanoma, for which the lowest known incidence is in Japan (2 cases in every 1,000,000 people every year) and the highest known incidence is in Queensland (309 cases in every x,000,000 people every year); the difference is 150-fold. This is attributable, at least in part, to the combination of light-skinned people exposed to very bright sunshine in Queensland near the equator compared with darker-complexioned people living in the less sunny clinics of Japan. Cancers arising in the back of the nasal cavity are common in Hong Kong (300 in every 1,000,000 people every year) but rare in the UK (3 in every 1,000.000 each year). The explanation for this difference is not so simple but may relate to chronic virus infection with a virus known as the Epstein-Barr virus, and may not exclude other factors such as dietary factors and genetic factors. The Chinese are not always on the wrong side of the equation. For instance, with cancer of the prostate gland in men 900 US blacks out of every 1,000,000 develop the disease each year but 13 in every 1,000,000 Chinese. We find the world’s highest incidence of stomach cancer in Japan (820 per 1,000,000 per year). This is to be compared with a much lower incidence elsewhere, particularly in the Middle East (3-4 per 1,000,000 per year).
The range for the common cancers can also be very large. The highest incidence of lung cancer is 1,100 per 1,000,000 per year in the US but only 58 per 1,000,000 per year in India; similarly colon cancer is 340 per 1,000,000 per year in the US and only 18 per 1,000,000 per year in India; breast cancer is 900 per 1.000,000 women per year in Hawaii but only 14 per 1,000.000 per year in parts of the Middle East. Where we are able to pin down the causes of these variations, the potential for prevention is obvious and large. Where the causes are unknown, the room for research it equally large and potentially equally rewarding.
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