Cancer prevention in humans may in general be grouped into two approaches: active and pas- sive. Table 13.1 supplies an outline of various methods of cancer prevention with an indication of the stage of carcinogenesis toward which the preventive measure is directed. The passive pre- vention of cancer involves the cessation of smoking, dietary restrictions, and modification of other personal habits such as those of a sexual nature. Active prevention of cancer development is usually accomplished by the administration of an agent to prevent infection by carcinogenic viruses and other organisms or by the intake of chemicals, nutrients, or other factors that may modify or prevent the action of carcinogenic agents. Theoretically, passive cancer prevention or the alteration of one’s “carcinogenic” habits can be the most effective and unintrusive method of cancer prevention. However, for many individuals, passive prevention requires external persua- sion, such as governmental regulation or peer pressure, to force an alteration of their habits. Obviously, in many instances such methods are doomed to failure. Active cancer prevention, which many consider a form of preventive “therapy,” is likely to be the most effective method in this area.
Most of the examples noted in Table 13.1 have been discussed earlier in this text. Individ- uals with hereditary conditions involving alterations in specific oncogenes or tumor suppressor genes constitute a relatively small part of the population. However, genes that may modify the susceptibility of an individual to the development of certain types of neoplasms probably represent significant factors in the development of an important fraction of human cancers (Spitz and Bondy, 1993). In reviewing the table, one can see that most methods of cancer prevention are linked to action at the stage of promotion. Because this is the reversible stage of neoplastic development, such a finding is not surprising. However, since we still do not know all or even most of the causes of human cancer, the continued identification of agents, especially chemicals, that might induce human cancer is important. While the results of epidemiological studies, when exhibiting sufficient evidence for a causal relationship, may be considered the “gold standard,” such detailed studies, even where feasible, for all the potentially carcinogenic agents existing and entering into our environment would be impossible. Therefore, during the last half century, as knowledge of the mechanisms of carcinogenesis increased, a significant effort backed by a
number of governmental agencies throughout the world was directed toward the development of methods for the identification of potentially carcinogenic agents in the environment by a variety of different systems from bacteria to whole animals. This chapter deals with the identification, characterization, and ultimate estimation of human risk from chemical, biological, and physical agents.