Cancer Clusters

Occupational and environmental cause vs. inevitable natural phenomenon

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This article, submitted by Anna M. Morales, won second place in the Student category of the 2004 RT Image Writing Competition.

One of the leading causes of deaths in America today is cancer, which is responsible for approximately one-fourth of the deaths in this country and costs the nation $180 billion a year in healthcare. However, the American Cancer Society (ACS) estimates that one-third of cancer deaths could be prevented. While cancer is a fairly common disease, a large number of cases quickly popping up in the same group of people is not. These cancer clusters may occur in a group of individuals in a common workplace or environment for a number of reasons. Prevention and control of cancer could improve if more states would make an effort to connect their cancer registry information to other sources of health information on a routine, readily accessible basis.

By connecting cancer statistical incidence data with other sources of health information, such as occupational and environmental risk or cause and personal risk factors, public officials could target situations of potential cancer clusters to reduce the overall incidence of cancer, not only in America, but around the world.

In order to identify the causes of the cluster, several areas must be identified. If an agent in the environment or a hazardous chemical in a work area were causing that cluster, the cluster's characteristics would likely follow guidelines.

States need to make an effort to link environmental and occupational cancer information to other health information sources to make it more accessible. Presently, states do not sufficiently provide data to the public or to researchers on cancer incidence by community in a timely manner. Each year there are approximately 1,000 inquiries from the public and organizations regarding cancer rates in their communities. Public and private organizations must therefore play this investigative role. It is rare, however, in these instances that investigations result in the discovery of true cancer clusters.

The Source

Upon first and continued exposure to a carcinogenic agent, cancer cells are stimulated to divide and become clinically recognizable. All cancers have varying latency periods, but a cluster is more likely to occur after a 15- to 20-year latency period. So first, the group of people in the cluster must have been exposed to the carcinogen for approximately 15 to 20 years prior to clinical presentation of symptoms.

Agents in a cluster typically cause the same form of cancer in the group of people. For example, radon exposure among miners has been linked to lung cancer, organochlorides found in pesticides has links to breast cancer and Agent Orange is associated with chronic lymphocytic leukemia among veterans.

Also, the type of cancer will occur in clusters among age and gender groups uncharacteristic of regularly occurring cancer. When several cases of a single cancer occur among a group with diverse ages and genders, it is most likely the result of an environmental agent. When different types of cancers appear in a workplace, it is unlikely that a cancer cluster of environmental or occupational cause is to blame; likely, the cancers have occurred by chance. Finally, if the suspected cancer is rare, an environmental cause may be to blame.

Cancer clusters can be difficult to identify for many reasons. First, cancer is usually caused by a combination of factors. Because cancer is common within any population, it is difficult to prove that a single agent has the power to induce it. Statistically, cancer will affect one in two men and one in three women in the United States. Therefore, it is not unusual to find cancer in the workplace. Because of the high probability of cancer among Americans, it is difficult to pinpoint environmental causes of cancer clusters, especially without adequate guidance from the state. The nature of cancer creates many scientific limitations. Because cancer so readily appears in a population, proving that it may have an environmental cause is difficult. This fact may be the most controversial aspect of cancer clusters. Cancer creates further limitations by having an unpredictable combination of causes.

According to the National Institute for Occupational Safety and Health, "Cancer is a group of different diseases that have the same feature," but, "each different type of cancer may have its own set of causes. Many factors play a role in the development of cancer. The importance of these factors is different for different types of cancer. A person's risk of developing a particular cancer is influenced by a combination of factors that interact in ways that are not fully understood." Therefore, because cancer can be unpredictable in its origin, even with today's technology, research trials cannot prove that any carcinogen present in the environment produces cancer 100 percent of the time.

When a cancer cluster is suspected, epidemiologists use their knowledge of incidence of that specific cancer type in a population to determine the validity of its suspected environmental cause. Epidemiologists use biostatistics to determine if that type of cancer really exists in excess within the specified population. The reference material utilized includes statistics of cancer incidence based on age, gender, ethnicity, family history, pre-existing medical conditions, diet and personal habits in comparison to frequency within other populations. They must also determine whether that type of cancer could have occurred by chance. Using tests to measure statistical significance, epidemiologists can verify the likelihood and probability of specific cancers to occur by chance in a population. In addition to statistics, scientists also assess a suspected cancer cluster by determining the cancer's origin. If the cancer's origin is not primary, it is not considered in a cluster investigation. Epidemiologists also utilize knowledge of the agent's history and its association with carcinogenicity. This knowledge about an agent's capability to produce certain cancers can confirm or deny the presence of a cluster.

Occupational Causes

In the United States, approximately 20,000 new cases of cancer are attributed to occupational cause each year. Cancers of the head and neck have been linked to exposure from everything from wood dust and nickel to radiation and asbestos in the workplace. Lung cancers can be attributed to asbestos, smoky work atmospheres and radon exposure. Studies on occupational settings have enabled researchers to educate the public on the importance of providing safety standards for prevention and elimination of cancer in the workplace.

In order to link a cancer cluster to workplace origin, epidemiologists evaluate various agents and their affect and existence in the environment to the general population. Exposure to arsenic, which causes lung and skin cancers, rarely occurs in the general public, but the poison is present in insecticide and herbicide sprayers, tanners and oil refineries. Formaldehyde is also rarely found in the general population but is known to cause nose and nasopharyngeal cancer in hospital and laboratory workers and in workers who manufacture wood products, papers, textiles, garments and metal products. Other carcinogenic agents found in the workplace include hair dyes and diesel exhaust and benzene – chemicals common in the general public that are associated with myelogeneous leukemia and lung cancer.

When carcinogenic agents are involved in a cancer cluster, the cluster could be attributed to occupational exposure. However, when the chemical has not yet been linked, it is more difficult to attribute and validate the cause for a cancer cluster. Undetected exposure can put many workers at risk without their knowledge. Unfortunately, in these situations, safety guidelines cannot be established and workers will eventually develop the hazardous effects of the exposure.

Environmental Causes

Studies of similar populations in different environments have provided essential data about the environmental influences on the etiology of cancer. Prior research acknowledges that a blend of factors influence the incidence of cancer. Cancers develop because of mutations within a gene. The degree of cancer risk increases, given the appropriate conditions in the environment and lifestyle.

Gene environment interactions are studies of large populations. These studies are required to validate previous theories that specific environmental exposures will result in cancer when specific genetic factors are introduced. Understanding the full impact of the environment on cancer is difficult because people within the same environment are unequally susceptible to such factors due to variation. Variables such as age, genetics and habits can adversely affect results. Nonetheless, environment does play a significant role on the susceptibility of a person to cancer.

Radon was established as an occupational carcinogen decades ago. Since then, the public's concern about exposure to radon has increased. Pipes, wires and pumps expose people to radon toxicity. Soil releases radon gas, which can enter and contaminate buildings through cracks. While most indoor exposure levels are lower than those experienced by occupational workers, the radiation levels of radon can become dangerous. In certain instances, radiation levels have exceeded the occupational limits of miners, exposing families to unsafe levels. Similarly, a study in an Italian alpine valley with high radon levels found that of 291 controls matched by gender and year of birth,138 died from lung cancer. Although these deaths were mostly associated with male smokers, the study does validate that environmental factors combined with other associated cancer risks increase the chance of cancer.

Age and Gender

Second only to cardiovascular disease, cancer is the leading cause of death for people age 65 and older. As age increases, so does the likelihood of cancer. In 2010, it is expected that 60 percent of all neoplasms will be among those 65 years of age and older. While the risk of cancer increases with age, the death rates as a result of cancer do not increase after the age of 85. While cancer still plays a significant role in the 65 and older age group, after this age, death is primarily a result of co-morbidities. People in this age group will more often die of other illnesses as opposed to their cancer.

According to a hypothesis in recent literature, the incidence of cancer increases with age because it is a chronic disease.

As discussed previously, cancer has a latent period, a lengthy development process and cells divide for years, and sometimes decades, before becoming a detectable mass. Because age is thought to be a function of susceptibility to cancer, as age increases, carcinogenesis is simply more likely in older tissue. Another important theory addresses environment – the older the population is, the more exposure they have had to carcinogens.

One in two men and one in three women will develop cancer over the course of their lifetime. In early 2003, the ACS estimated 675,300 new cases of cancer in males and 658,800 in females, suggesting that males are at an increased risk for cancer in comparison to females. Males had almost double the incidence of estimated head and neck cancers in 2003 (18,200 vs. 9,500). Of the 55 categories in new cancer cases, males dominated 46 categories. Of these same 55 categories, males had significantly higher occurrences of cancers of the tongue, pharynx, esophagus, liver, skin, genital system and leukemia. Categories in which females had a significantly higher estimated risk than males include breast, colon, anus, pancreas and endocrine (thyroid) cancers.

Ethnicity and Economics

While there is no reason to believe that cancer is predetermined in any one specific racial or ethnic group, several factors contribute to an increased risk of cancer based on ethnicity and race. Among studies on the genetic mutations that predetermine cancer, all races and ethnicities exhibit many similar features and are capable of acquiring the same types of cancers. Differences between culture and ethnic-related cancer risk occur because of dietary habits, socioeconomic status and cultural habits. For example, alcohol and tobacco products have, for many years, been linked to various cancers and show varying patterns of use by economic status, social situation and culture. Generally, those of lower economic status gravitate toward smoking. This not only means an increase among specific groups but its relevance has also been shown to produce a synergistic effect in regard to cancer-creating mechanisms. Studies predict the less educated tend to more readily adopt alcohol and tobacco use in their daily habits. African-American males also have the highest death rates from all cancer sites. Death rates from cancer among African-American males are 1.4 times higher than those of Caucasian males.

The role of socioeconomic factors, such as poverty, inadequate education and lack of health insurance, on the influence of cancer seems to be more significant than biological differences. Other factors of associated cancer risk due to socioeconomic factors include poor nutrition, physical inactivity and obesity. Those living in poverty do not generally have access to pricey fresh fruits and vegetables as recommended by the ACS.

Studies also show that culture has an impact on people who migrate to a new culture. An outsider will eventually adopt habits of this new culture, therefore transitioning from an area of low risk to areas of high risk can increase risk of the disease. However, some incidences of cancer can be attributed to environmental factors of a particular geographic region alone. For example, a study of the inhabitants of Linxian, a county in China, revealed one of the highest rates for esophagus squamous cell carcinoma in the world. After many studies, alcohol and tobacco consumption, genetics and possible causative agents were not to blame. The study showed this disease was related to the Epstein-Barr virus, which had increased in this location due to a malaria endemic.

When all factors of ethnicity and socioeconomic status, in conjunction with dietary personal habits, are accounted for, white, non-Hispanic females have the highest rate per 100,000 of breast cancer (1990-1995). Black females have the highest rate of lung and bronchus and colon cancers (1990-1995). Asian-Pacific Islander males and females have the highest rates in stomach and liver cancers (1990-1995). Finally, black males have the highest rates for prostate, lung and bronchus, colon, oral cavity and pharynx cancers (1990-1995).

Family History and Pre-Existing Medical Conditions

Of the human cancers that exist, only 5 percent are accounted for by genetics. Mutations of DNA repair genes account for some inherited cancer syndromes. Breast cancer is one example of a hereditary cancer.

Viruses have been known to cause mutations and code for cancer growth. The human papillomavirus, Simian Virus 40 (SV-40) and herpes simplex virus (HSHV) are a few pre-existing medical conditions that have been linked to the formation of cancer. Originally, SV-40 was discovered in rhesus monkey cells. It contaminated the poliomyelitis vaccine. Unaware of this virus, scientists and physicians injected the vaccine into millions of people. Laboratory rodents developed brain and pancreatic endocrine tumors among others.

Among the viruses linked to cancer is the Epstein-Barr virus. This virus has been linked to nasopharyngeal cancer. Populations of AIDS patients around the world also have a high incidence of malignancies. Forty percent of AIDS patients have had a detected malignancy, the most common being Kaposi's sarcoma (KS). This malignancy is suspected to be associated with the HIV infection and may also be linked to KSHV. Kaposi's-Sarcoma-Herpes virus is found in 90 percent of AIDS-KS tumors.

Two Cents

Cancer cases are not produced by one single environmental factor because personal risk factors act in conjunction with each other. However, it should not be concluded that occupational or environmental exposure does not exist. This is the most controversial part about cancer clusters. Without the presence of toxins, agents or other chemicals, cancer can be avoided. Perhaps the missing link in many cases of risk factors is a single agent. Without the presence of that single agent, a cancer is one degree away from infecting a body system and perhaps with the presence of that agent, the cancer possibility is made whole. Unfortunately, the limitations of research regarding the origin of cancer are the inability to place the blame on one cause.

It's impossible to eradicate occupationally linked cancer, but employers should take responsibility to establish appropriate safety guidelines. Furthermore, employers should use current literature and the implications of cancer causes and use them to their full advantage. While guidelines exist in many occupations, education provides the best protective barrier against cancer. Through education, employees know the risks they take when working daily with toxic agents.

Knowledge provides the wisdom to avoid dangerous chemical exposure combinations that may have a synergistic cancerous effect. It's important to use safety in handling toxic agents. The strengths of this data and current literature reveal a definite link between instances of cancer and many chemicals, but if handled correctly, these chemicals will not cause an otherwise healthy person to develop cancer.

Ultimately, cancer clusters of occupational and environmental cause cannot be disregarded. States need to take more responsibility. With technology today, real-time reporting and statistics about cancers should occur on a city, county and statewide level via the Internet. Genuine, high-quality data must be systematically obtained to enhance quality control. Further research by the state should be enforced in the effort to connect occupational and environmental risk/cause and personal risk factors. This type of system exposes people within a population to patterns and trends that are occurring in their neighborhoods. It can be accessible to everyone while maintaining patient confidentiality. Providing knowledge to the citizens of a city can affect the entire nation and, perhaps, one-third of cancer deaths could be prevented.

By learning to recognize risk factors and hazardous situations, researchers, scientists and epidemiologists can help the general public lower their risk. People should take risk assessment surveys and become more responsible for their health. Not only can gaining a better understanding of risk factors save lives, but it can also help researchers come to realize the most hazardous combinations of risks; only then can the assessments they employ give the general public a better understanding of the nature of cancer clusters.

— Anna M. Morales is a senior in the radiation therapy program at Texas State University, San Marcos. Questions and comments can be directed to editorial@rt-image.com.

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