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Chapter 43

Geographic Variations of Malignant Neoplastic Diseases in the Tropics

There is no simple answer to the question of why there are wide variations in the type and frequency of malignant disease in various parts of the world. Some malignant disease is known to be familial (e.g., polyposis of the colon and its subsequent malignant degeneration), but genetic traits are another cause. The incidence of malignant disease among the black people of America is far closer to that among their white neighbors than it is to the incidence in black Africans. Unfortunately, "Africans" are not one people, but vary in every way across the continent: every type of disease, including malignancy, has local variations, not only over large distances but sometimes within a few kilometers (or miles). Indeed, the pattern of malignancy seen in the black Africans in West Africa is, despite many differences, closer to that in black Americans than it is to that in Africans in East and southern Africa. In many countries, not only in Africa but, for example, in Singapore, it is necessary to separate the population into distinct groups, such as the Chinese, Malays, or Indians, before describing the incidence of particular tumors. All over the world in major countries, languages, customs, philosophy, geography, and disease vary infinitely and are probably intricately but significantly interrelated.

Geographic variation also occurs because many malignant tumors are influenced by the environment. For example, the high incidence of cancer of the stomach that occurs in Japanese who live in Japan is not equaled in Japanese who live in Hawaii, and the rate of breast cancer in Japanese women who migrate to Hawaii alters to equal that of the host country within one or two generations. "Environment" in this context covers not only differences in geography and climate, but variations in cultural habits and socioeconomic status. To these differences must be added variations in the chemicals we eat, breathe, and use, viruses, and no doubt other factors which are unrecognized.

Malignant disease in the tropics often differs from that elsewhere. There are many tribes in Africa in which the men are uncircumcised and yet there is a low incidence of carcinoma of the penis. Bladder cancers in Egypt and Uganda are histologically almost all squamous cell rather than transitional cell carcinomas. It may seem simple to ascribe the majority of these bladder cancers to the high incidence of schistosomiasis haematobium in Egypt, but the cancers are also common in Uganda, where that parasitic disease is not so prevalent. Even the frequency of cancer differs in the tropics: it does not iincrease in the same way with old age. There is increased mortality in the tropics, so that the population as a whole does not reach the same average age as in temperate and more developed countries: but even if this is taken into consideration, malignant disease in the tropics is less common in older age groups. In the elderly, cancer is ten times more frequent in New York than in Ibadan (Nigeria), yet childhood malignancy is much more common in the tropics than in the temperate climates. The environment may be killing the elderly Americans, but what is it that the children of Africa have, or lack, that has the same effect?

Overall, the incidence of malignancy in the tropical countries is less than 50% of that observed in the rest of the world, but the majority of cancers, perhaps 80% or 90%, are likely to be incurable by the time the diagnosis is made. Some tumors are many times more frequent in the tropics: for example, carcinoma of the cervix is four times more common in Uganda than in New York and twice as common in Uganda as in southern Africa. In many parts of the tropics, the ratio of invasive cancer of the cervix uteri to cancer of the corpus uteri is 20:1. In more temperate climates the difference is less striking.

There is no doubt that much can and will be learned from the data on malignant disease in the tropics which have been gathered in the last 50 years. Methods of comparing different populations with different age and sex ratios are becoming more reliable, and, hopefully, more statistically significant. Unfortunately, the very important cancer registries which were established in major centers in Africa, and in some other parts of the world, in the 1950s and 1960s are not all working today, or are not maintaining the required standard. The cost of these centers is very high and, apart from this, the social climate will no longer accept the high autopsy rate which is necessary to establish reliable data.

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