Cancer etiology

The etiology of cancer can be viewed from two perspectives: its molecular origins within individual cells and its external causes in terms of personal and community risks. Either viewpoint is complex. Together, these perspectives form a multidimensional web of causation by which cancers arise from the interplay of causal events occurring in tandem over time.
1. HOST FACTORS
Though considering cancer causation in terms of intrinsic and extrinsic factors (i.e., host and environment) is common, drawing the distinction is not always easy. Taken in their entirety, host risk factors may be considered conveniently within three major categories: inherited genes, hormones, and immunologic mechanisms. Each instance, however, allows ample opportunity for interplay with environmental forces. Therefore, frequent consideration must be given to the potential mutagenic effects of environmental chemicals and radiation, or to the cellular impact of infectious agents and exogenous hormones, when host-oriented etiologic judgments are being formed.
2. ENVIRONMENTAL CAUSES
Estimates support that 75% or more of cancers are the result of environmental exposures. This conclusion arises from rate comparisons based on the recognition that site-specific cancer rates differ very widely from place to place in the world and that persons migrating from one country to another tend to develop rates similar to those in their new country. Environmental exposures, of course, encompass all influences arising outside the host; therefore, they include the many carcinogenic exposures associated with personal lifestyle and behavior (e.g., diet, tobacco use) in addition to those arising in the general community and in workplace environments.
3. LIFESTYLE ENVIRONMENT
As suggested, individual chemicals, genes, or physical agents do not by themselves provide a full picture of cancer causation. Single causes do not act alone. Instead, our experience with them occurs in settings in which many causal elements are present and interact. This condition is particularly true of the two most important areas of environmental cancer causation—tobacco use and dietary habits—each of which involves many different agents that can influence the process of carcinogenesis. For all categories of environmental risk, however, whether related to lifestyle or to community or workplace environments, epidemiologic research, our chief source of human evidence, regularly weighs the impact of selected risks against a wide array of competing or modifying variables. As our knowledge grows regarding the etiology of cancer, new opportunities may arise by which we can develop new or improved methods for its effective prevention or control.