August 2, 2011

Around 400 BC, Hippocrates described the long, bulky veins spread around some breast tumors as resembling the limbs of a crab, hence its name—karkinoma in Greek and, later, its Latin equivalent cancer. Although cancer is not one disease, all cancers share one characteristic and it is the uncontrolled growth of cells.

Our normal body is built of 30 billion cells. Each healthy cell while restrained to a defined space—with the exception of blood cells circulating in the body— reproduces (proliferates) and dies (apoptosis) in a meticulous concord with itself and other cells to keep the body as a whole. Tumors arise when the process of reproduction loses its harmony. Thus, cells proliferate without control and form tumors. Tumors are called benign or in situ if they remain in the assigned boundaries (tissue). If tumor cells acquire an additional capacity of breaking the tissue in which they belong, they become malignant and further capable of invading other tissues. Offspring of malignant cells travel through blood or lymph vessels and lodge in a distant site. Invasion of distant sites by cancer cells originated in other sites (primary sites) is called metastasis. Most cancer deaths are the result of metastasis.

The process of cell reproduction is complex yet controlled mainly by two groups of genes, protooncogenes and tumor suppressor genes. These genes together with all the other genes are carried in the DNA molecules in the core of each cell (nucleus). Protooncogenes are responsible for the stimulation of growth and their activities result in reproduction and cell division. Mutation of these genes therefore will result in overgrowth of cells. Ras and myc gene families are examples of proto-oncogenes involved in cancers of breast, lung, colon, and leukemias. Tumor suppressor genes, in contrast, are an obstacle to improper cell division. Nonfunctional tumor suppressor genes will allow inappropriate cell reproduction. BRCA1, BRCA2, and P53 are examples of tumor suppressor genes involved in breast and ovarian cancers. Although mutation in these two groups of genes contributes remarkably to the development of cancer, involvement of other muted genes together with other, yet unknown, mechanisms may also be important for tumors to become malignant and further detach from its residence and spread to other sites.

In contrast to infectious disease where the time between exposure to the infectious agent and the occurrence of disease (incubation period) is usually measured in days, weeks, or months, the latency period—the time from exposure to agents causing or contributing to cancer (carcinogens)—for cancer can be estimated up to decades. Transformation of normal cells to cancerous cells requires a relatively long process of accumulation of several muted genes over time. Individuals with inherited (carried from parents to children) muted genes develop cancer at a younger age because they require less time for the carcinogenic process to be complete.

Apart from cancers caused by inherited mutations in genes, which accounts for about 5% of all cancer deaths, most cancers are caused by factors related to individuals’ environment and lifestyle and are hence modifiable. Epidemiological studies investigating cancer occurrence in relation to the migration of human populations have delineated the relative importance of these factors on cancer occurrence. While genetic compositions remain constant upon migration, environmental and lifestyle factors are subject to change. One evidence of the major role of environmental and lifestyle factor in cancer is the observed variation in the risk of several cancers across socioeconomic groups. Cancers of the breast, prostate, colon, and melanoma are more common in affluent than in underserved populations. Some of the factors associated with differences in cancer incidence across socioeconomic groups are reproductive behavior (i.e., late age at complete pregnancy and less number of children associated with increased breast cancer risk) and nutrition (i.e., consumption of red meat and saturated fat increase risk for colon cancer). The opposite is true for cancers of the cervix, stomach, liver, esophagus (throat), and lung, which affect people of lower socioeconomic status at a disproportionately higher rate. The prevalence of some unhealthy lifestyle factors such as smoking (cancer of the cervix, lung, and esophagus), heavy alcohol consumption (liver cancer), unsafe sexual behavior (cancer of the cervix), and infection with cancer-causing viruses and bacteria (Helicobacter pylori and stomach cancer) associated with several cancers seems to be higher in less affluent populations.

Preceded only by heart disease, cancer is the second leading cause of death in the United States. It was estimated that in 2003, over 1.3 million new cases would be diagnosed with cancer, and over half a million deaths would occur as a result of cancer. Leading sites of new cancer cases are prostate (33% of all new cases), lung and bronchus (14%), and colon and rectum (11%) in men; and breast (32%), lung and bronchus (12%), and colon and rectum (11%) in women. In both men and women, lung and bronchus is the leading site for cancer-related deaths. The cancer-related economic burden to the society is substantial, with over $189 billion in direct medical costs and lost productivity due to the disease and to premature death, as estimated for 2002.

SEE ALSO: Access to health care, Agricultural work, Breast cancer, Cervical cancer, Disparities in Women’s Health and Health Care (pp. 13-20), Health insurance, Health maintenance organizations, Smoking, Women in the Workforce

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Category: C, Cancer