Epidemiology of cancer

DEFINITION: Cancer epidemiology studies the distribution, determinants, and frequency of malignant disease in specific populations. Three broad objectives related to the use and importance of epidemiologic cancer research are: identifying factors related to particular cancers' etiology, using cancer-risk data to formulate theories regarding cancer prevention and control, and educating a diverse public regarding the prevention and control of cancer. These strategies attempt to match educational and dissemination methods to population needs with attention to cancer risk, literacy, trust of medical professionals, and palatability of information, factors known to vary widely across population subgroups.

Three types of epidemiologic research apply to the field of cancer. Descriptive epidemiology focuses on the trends and frequency of cancer in specific populations. Analytic epidemiology identifies causes and predisposing risk factors associated with developing specific cancers. Clinical epidemiology delineates screening programs and evaluates outcomes for the prevention strategies implemented.

Descriptive epidemiology: Cancer incidence and mortality rates are higher among men than women. Furthermore, Black Americans have a higher cancer mortality rate than Whites, which has been a source of discussion and controversy. Some argue that inherent genetic factors account for the discrepancy, whereas others attribute the difference to an overrepresentation of poverty among Black Americans and superior healthcare received by affluent versus impoverished groups.

The leading cancers in the United States vary slightly for men and women. Leading cancers for men include prostate, lung and bronchus, colorectal, urinary bladder, and melanoma of the skin. For women, the leading cancers include breast, lung and bronchus, colorectal, ovarian, cervical, skin, and thyroid. Individuals over sixty-five have a significantly greater risk of developing cancer than younger Americans. Despite an increase in overall cancer mortality rate between 1950 and 1990, mortality rates for all cancers combined have declined substantially for individuals under age forty-five. They increased in people over age fifty-five, with that increase primarily related to lung cancer deaths, but began declining in the mid-2000s.

These data seem biased and problematic, as smoking is the primary etiologic factor in lung cancer diagnoses. Excluding lung cancer from cancer death statistics implies that if lifestyle issues and behaviors are related to etiology, science is not responsible or interested in research to develop effective prevention and treatment strategies. This denies the role of social, political, and economic factors in promoting increasingly prevalent maladaptive behaviors among vulnerable members of society and is an area worthy of aggressive debate and action regarding research funding.

Analytic epidemiology: The goal of analytic cancer epidemiology is to identify the factors predisposing individuals to a cancer diagnosis and quantify risk. Cancer risk factors include environmental exposures, genetic susceptibility, and immunosuppressive state and may be secondary to a history of malignancy, viral infection, or medical therapy. These risk factors can account for various aspects of carcinogenesis and assume varied degrees of causal primacy.

Clinical epidemiology: Epidemiologic research is vital in developing cancer screening modalities and prevention strategies. Cancer prevention focuses on decreasing incidence by lowering risk through lifestyle and behavior changes. Primary prevention attempts to stop the development of cancer. Secondary prevention aims to improve cure rates through cancer screening, early diagnosis, and treatment.

Cancer screening involves testing to detect early-stage cancer in asymptomatic individuals. Screening tests should be easy to administer, noninvasive, and inexpensive. To be beneficial, early detection should alter prognosis and improve survival.

Emerging data and discovery: Data and research continually emerge concerning cancer screening and prevention practices. Although advancements are important, the proliferation of data can create confusion among consumers, and avenues for reliable information are needed.

For example, the human papillomavirus (HPV), types 16 and 18, has been causally related to cervical intraepithelial neoplasia. A history of genital warts is linked to human papillomavirus types 6 and 11 and may explain the increased risk associated with multiple sexual partners. Other sexually transmitted viruses, such as herpes simplex virus 2, may interact as etiologic factors. The vaccines Gardasil and Cervarix protect against four HPV types, which cause 70 percent of cervical cancers and 90 percent of genital warts. Girls and young women should get the vaccine before they are sexually active because the vaccine is most effective in those without HPV. Girls and women who have not been infected with HPV will get the full benefits of the vaccine. Those who are sexually active may also benefit from the vaccine, although they may get less benefit if they have already acquired one or more HPV types.

Global application of the vaccine could reduce the annual mortality rate of cervical cancer, but this technology raises questions and obstacles. The need to vaccinate women before they become sexually active raises ethical issues, particularly among populations that fear the vaccine will send a message condoning teenage sexual activity. Other issues revolve around who should advertise and promote the vaccine and how it should be promoted in developing nations. These and many other issues demonstrate how medical advances complicate cancer epidemiology and offer hope.

Sociocultural issues: As research and technology advance, ethical questions continue to emerge, such as fair and equitable allocation of health resources across populations, prioritizing individual needs versus the social aggregate, and the unfair distribution of resources to privileged groups. Some of these issues have become especially poignant as genetic technologies have identified specific ethnic factors to enhance the risk for certain cancers. The incidence increases fiftyfold in Whites and thirtyfold in Black Americans between the ages of fifty and eighty-five. Black Americans have the highest incidence of prostate cancer, whereas men of Asian/Pacific Islander descent have the lowest rates. Black American men tend to have metastatic disease at diagnosis. The overall survival rate for Black American men is 10 percent lower than that for White men, even when diagnosed at the same stage. Endometrial carcinoma is the most common gynecologic malignancy. Its incidence is highest among White women, but its mortality rates are higher among Black women. Cultural, psychosocial, and demographic factors may discourage the use of genetic testing services among individuals who could benefit from them or, conversely, may promote use when testing has the potential to create more harm than benefit.

To develop effective cancer detection and prevention programs, it is essential to consider cultural, demographic, and psychosocial issues that may foster or hinder utilization. Consumer-led advocacy groups have taken a grassroots approach to soliciting research dollars to fund basic and clinical research programs. That fact alone may skew inquiry toward those cultural, ethnic, and socioeconomic subgroups who value and can interpret medical research results and have sufficient personal resources to advocate on their own behalf. Lack of interest in genetic testing has been associated with less education, minority status, lower socioeconomic status, and less performance of other health-promoting behaviors.

Culture plays a central role in determining health beliefs, attitudes, and behavior, but only some healthcare providers realize that health is a cultural concept defined differently across cultures. Many are unaware that the healthcare system is culturally designed and administered largely according to mainstream values. In genetics, cultural consideration is critical. Although the knowledge that certain diseases run in families spans all societies, beliefs about the causation of familial diseases vary considerably. Cultural attitudes toward disease also differ among ethnic populations. Culture comes into play in provider-client interaction and communication, both critical components of genetic counseling. People from cultures that expect authority figures to be directive may find nondirective genetic counseling confusing and bewildering.

Although cultural competency is not a licensing requirement for health professionals, it is vital given the sensitive nature of the issues to be discussed and the need for privacy and confidentiality of information exchanged. The professional genetics community includes a few minorities despite demographic trends limiting their access to culturally sensitive and relevant genetic services. This lack of input from minority communities also limits the shaping of public policy and planning of gene research and counseling in ways meaningful to ethnic minorities.

Genetic services must be available, culturally appropriate, accessible, and affordable for individuals to benefit from them. Unless such issues are addressed, any attempt to broaden access to genetic services will be limited and hazardous to a less informed population. Only some providers are prepared to assume these tasks. They need education on the scientific advances in genetics and their ethical, legal, cultural, and psychosocial implications.

Inadequate appropriations to develop the needed service infrastructure have resulted in insufficient genetic specialists, primary care, and public health providers prepared to incorporate genetics into practice. Among primary care providers, inadequate genetic preparation is compounded by the severe time constraints imposed on service providers and a lack of reimbursement for cognitive services.

Most consumers also need higher scientific literacy and more about basic genetics or genetic testing. Without gene literacy and an understanding of the limitations and risks involved, including insurance and employment discrimination, psychological trauma, intrafamilial conflict, and social stigmatization, people cannot make truly informed decisions.

Chemoprevention: A modern approach to cancer prevention is through chemoprevention (as opposed to chemotherapy or drug treatment following a cancer diagnosis). Cancer chemoprevention is defined as the reversal of carcinogenesis in the premalignant phase. The observation that retinoids, acting as modulators of cell differentiation, are effective in suppressing oral carcinogenesis and, therefore, in preventing second primary tumors in squamous cell carcinoma of the head and neck has led to the evaluation of these agents as chemopreventive therapy for tumors of the upper aerodigestive tract in high-risk populations. Studies of adjuvant hormonal therapy with tamoxifen for breast cancer have shown a 50 percent reduction of contralateral disease, which led to a national tamoxifen chemoprevention trial to evaluate risk reduction for primary breast cancer in women at high risk. With modern molecular techniques, chemoprevention trials will be aided by identifying markers for premalignant lesions.

Prognosis for cancer epidemiology: Decision-making regarding genetic susceptibility testing, health surveillance, chemoprevention, and preventative surgery are being quickly added to healthcare options without adequate knowledge among professionals and the public regarding the meaning and practical relevance of this information. Psychosocial, cultural, and economic factors may affect the study, dissemination, and utilization of genetic discoveries to create service barriers and widen the gap between the socioeconomic classes of healthcare consumers. Genetic information is received against a backdrop of deeply held personal beliefs. The influence of culture on health beliefs and actions is enormous, and the role that culture, ethnicity, and religion play in formulating an individual’s motivation toward health-seeking behaviors must be addressed in all educational and clinical activities. 

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