CANCER: EPIDEMIOLOGY - HOW CONCLUSIVE? (part 1)

We have outlined the ways in which epidemiologists can describe and analyse the vast amount of information now being collected about human cancer in populations. Can they produce conclusions from these studies? The answer is: probably not from any single study. Only the intervention studies can generate precise answers to precise questions, and these are few and far between and very difficult to perform adequately. In most other situations, conclusions are reached by adding together the suggestions derived from the descriptive and the analytical studies, often when several such studies have been performed. In addition, facts that emerge from experimental science in the laboratory or from clinical sciences often need to be built into the overall equation before conclusions can be reached.

Why are single descriptive studies often fallible? Even if we find that one population or group is more prone to cancer than another, we may have considerable difficulty in determining which factor (or combination of factors) is responsible for the higher incidence of the disease. It is very unlikely that the two populations or groups will be different from each other in only one respect or in a very limited number of ways. The number of variations between them (both environmental and generic) is likely to be enormous. The population which is more exposed to the risk (or combination of risks) in which the cancer epidemiologist is interested may be different from the other population in a host of other ways that could just as easily account for the higher incidence of cancer. A cancer may have many interacting causes, some of which are, as yet, unknown. The epidemiologist may thus overlook some of the variations between the exposure characteristics of the two populations which might explain the different rates of cancer.

The ideas that we have to teat in epidemiology have to come either from initial observations in populations (like the connection between smoking and Jung cancer, or between occupation and certain cancers) or from the laboratory. We often do not have precise enough ideas to test, and some of our ideas may be wrong. We are fortunate when a clinician comes up with a due like that which occurred for Percival Pott from his observations of chimney-sweeps. Similar astute clinical observations have led on to other detailed epidemiological studies which have reached helpful conclusions. The link between bone cancer and the use of radioactive substances in manufacturing was first described this way, as was the link between asbestos and the very serious form of chest cancer known as mesothelioma.

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Cancer