We can apply the growing understanding of the molecular biology of cancer to sharpen our attack on the disease at three levels: prevention, diagnosis, and treatment. Prevention is always better than cure, and indeed many cancers can be prevented, especially by avoiding smoking. Highly sensitive molecular assays promise new opportunities for earlier and more precise diagnosis, with the aim of detecting primary tumors while they are still small and have not yet metastasized. Cancers caught at these early stages can often be nipped in the bud by surgery or radiotherapy, as we saw for colorectal polyps. Nevertheless, full-blown malignant disease will continue to be common for many years to come, and cancer treatments will continue to be needed.
In this section, we first examine the preventable causes of cancer and then consider how advances in our understanding at a molecular level are beginning to transform the treatment of the disease.
Epidemiology Reveals That Many Cases of Cancer Are Preventable
A certain irreducible background incidence of cancer is to be expected regardless of circumstances. As discussed in Chapter 5, mutations can never be absolutely avoided because they are an inescapable consequence of fundamental limitations on the accuracy of DNA replication and repair. If a person could live long enough, it is inevitable that at least one of his or her cells would eventually accumulate a set of mutations sufficient for cancer to develop.
Nevertheless, environmental factors seem to play a large part in determining the risk for cancer. This is demonstrated most clearly by a comparison of cancer incidence in different countries: for almost every cancer that is common in one country, there is another country where the incidence is much lower. Because migrant populations tend to adopt the pattern of cancer incidence typical of their new host country, the differences are thought to be due mostly to environmental, not genetic, factors. From such findings, it has been suggested that 80–90% of cancers should be avoidable, or at least postponable (Figure 20–37).
Unfortunately, different cancers have different environmental risk factors, and a population that escapes one such danger is usually exposed to another. This is not, however, inevitable. There are some human subgroups whose way of life substantially reduces the total cancer death rate among individuals of a given age. Under the current conditions in the United States and Europe, approximately one in five people will die of cancer. But the incidence of cancer among strict Mormons in Utah—who avoid alcohol, coffee, cigarettes, drugs, and casual sex—is only about half the incidence for non-practicing members of the same family or for Americans in general. Cancer incidence is also low in certain relatively affluent populations in Africa.
Although such observations on human populations indicate that cancer can often be avoided, it has been difficult in most cases—with tobacco as a striking exception—to pinpoint the specific environmental factors responsible for these large population differences or to establish how they act. Nevertheless, several important classes of environmental cancer risk factors have been identified (Figure 20–37B). One thinks first of mutagens. But there are also many other influences—including the amount of food we eat, the hormones that circulate in our bodies, and the irritations, infections, and damage to which we expose our tis-sues—that are no less important and favor development of the disease in other ways.
Many quite disparate chemicals are carcinogenic when they are fed to experimental animals or painted repeatedly on their skin. Examples include a range
SOUTH AMERICA NORTH AMERICA HAWAII JAPAN CHINA AFRICA prostate, colon, breast prostate, colon, breast stomach naso-pharyngeal Burkitt’s lymphoma Burkitt’s lymphoma (A) EASTERN EUROPE Hodgkin’s disease
Figure 20–37 Cancer incidence is related to environmental influences.