Mechanisms of Carcinogenesis

Abstract

Epidemiologists John Higginson and Sir Richard Doll have estimated that 80%–90% of human cancers result from environmental factors.This estimate was based on the comparison of an individual’s average risk for the development of cancer to that obtained by summing up the lowest observed cancer rates for each organ site. In the context of this estimate, the environment is broadly defined and includes the direct induction of cancer by exposure to specific chemicals or viruses as well as the modification of cancer risk by dietary factors or reproductive patterns. Exposure to carcinogenic chemicals may result from social habits such as tobacco usage, the ingestion of some naturally occurring toxins, or work in certain industries. Members of four families of human viruses have also been causally associated with the development of specific neoplasms. As the risks for cancer development at various sites show wide geographic variation, so, too, does the relative importance of these etiologic agents vary from place to place. For example, tobacco consumption accounts for approximately 30% of all cancer deaths (principally lung cancer) in North America, while primary hepatocellular carcinoma associated with hepatitis B virus infection is responsible for up to 25% of cancer fatalities among males in parts of Africa and Asia. Further details of the spectrum of environmental exposures known or suspected of being etiologic in human cancers are presented in the chapter “Descriptive Epidemiology.”