What are cancer clusters?
Facts about cancer clusters
Heredity and environment
Geographic patterns
Cancer cluster reporting
Getting information about cancer clusters
What are cancer clusters?
A disease cluster is the occurrence of a greater than expected number of cases of a particular disease within a group of people, a geographic area, or a period of time. Cancer clusters may be suspected when people report that several family members, friends, neighbors or coworkers have been diagnosed with the same cancer or related cancer. In the 1960s, one of the best-known cancer clusters emerged, involving many cases of mesothelioma, a rare cancer of the lining of the chest and abdomen. Researchers traced the development of mesothelioma to asbestos exposure. Working with asbestos, which was used heavily in shipbuilding during World War II and has also been used in manufacturing industrial and consumer products, is known as the major risk factor for mesothelioma.
Facts about cancer clusters
Reported disease clusters of any kind, including suspected cancer clusters, are investigated by epidemiologists, scientists who study the frequency, distribution, determinants and control of diseases in populations. Epidemiologists use their knowledge of diseases, environmental science, lifestyle factors, and biostatistics to try to determine whether a suspected cluster represents a true excess of cancer cases.
Epidemiologists have identified certain circumstances that may lead them to suspect a potential common source or mechanism of carcinogenesis among people thought to be part of a cancer cluster. A suspected cancer cluster is more likely to be a true cluster, rather than a coincidence, if it involves:
- A large number of cases of a specific type of cancer, rather than several different types
- A rare type of cancer, rather than common type
- An increased number of cases of a certain type of cancer in an age group that is not usually affected by that type of cancer
Before epidemiologists can accurately assess a suspected cancer cluster, they must determine whether the type of cancer involved is a primary cancer or a cancer that is the result of metastasis (spread from another organ). This is important because scientists consider only the primary cancer when they investigate a cancer cluster.
Epidemiologists also try to establish whether the suspected exposure has the potential to cause the reported cancer, based on what is known about that cancer's likely causes and what is known about the carcinogenic potential of the exposure. Scientists use various statistical methods to determine whether the reported excess of cases is really a larger number than would normally be expected to occur.
Before a cluster can be considered "true" epidemiologists must show that the number of cancer cases which have occurred is significantly greater than the number of cases that would be expected, given the age, gender, and racial distribution of the group of people at risk of developing the disease. However, it is often very difficult, if not impossible, to accurately define the group of people who should be considered at risk.
Heredity and environment
Because most cancers are likely to be caused by a combination of factors related to heredity and environment (including behavior and lifestyle), studies of suspected cancer clusters usually focus on these two issues. However, establishing significant and valid evidence that a specific genetic factor leads to an increased chance that a specific environmental exposure will result in cancer (called a gene-environment interaction) requires studies of large populations over long periods of time. Researchers are just beginning to unravel the puzzle of carcinogenesis in terms of the roles of heredity and environmental exposures. Some of their discoveries are outlined below:
Heredity:
- All cancers develop because of genetic mutations of one kind or another.
- Some alterations that increase the risk of cancer are present at birth in the genes of all cells in the body, including reproductive cells. These alterations can be passed from parent to child. This is known as an inherited susceptibility. This type of alteration is uncommon as a cause of cancer.
- Most cancers are not due to an inherited susceptibility but result from genetic changes that occur during one's lifetime within the cells of a particular organ.
- Familial cancer clusters (multiple cases among relatives) have been reported for many types of cancer. Because cancer is a common disease, it is not unusual for several cases to occur within a family.
- Familial cancer clusters are sometimes linked to inherited susceptibility, but environmental factors and chance may also be involved.
- Having an inherited susceptibility for a type of cancer does not guarantee that the cancer will occur; it means there is an increased chance of developing cancer if other factors are present, or later develop, which promote the development of cancer.
Environment:
- The term environment includes not only air, water, and soil, but also substances and conditions in the home and workplace. It also includes diet; the use of tobacco, alcohol, or drugs; exposure to chemicals; and exposure to sunlight and other forms of radiation.
- People are exposed to a variety of environmental factors for varying lengths of time, and these factors interact in ways that are still not fully understood. Further, individuals have varying levels of susceptibility to these factors.
- Because workers may have heavier and more prolonged exposures to hazardous chemicals that are found widely distributed at lower levels in the general environment, positive findings from studies in the workplace provide important leads regarding causes of cancer in other settings. In fact, occupational studies have identified many specific chemical carcinogens and have provided direction for prevention activities to reduce or eliminate cancer-causing exposures in the workplace and elsewhere.
Geographic patterns
The National Cancer Institute has compiled an atlas of cancer mortality in the United States, using 1950 to 1994 mortality data from the National Center for Health Statistics and population estimates from the Census Bureau. The map shows that the patterns previously observed in an earlier mapping for several cancers have persisted, such as the broad stretches of high rates for cancers of the breast, colon and rectum in the Northeast. (Rates are also rising in many areas of the South.)
For lung cancer, the map shows high mortality rates among white men across the South, among white women in the far Western states, and among blacks in northern urban areas. The study of geographic variation in cancer rates is providing important clues to the role of lifestyle and other environmental factors that affect cancer risk.
Cancer cluster reporting
Most states currently have central registries that collect data on the number of new cancer cases reported. The data in these registries can be used to compare expected cancer rates in certain categories, such as a geographic area, age, or racial group, with rates reported in a suspected cancer cluster to determine whether there is a true excess of cases. State health departments often receive assistance from a number of federal agencies, including the Centers for Disease Control and Prevention, the Agency for Toxic Substances and Disease Registry, and the Environmental Protection Agency.
Getting information about cancer clusters
Local and state health departments are listed under such headings as "health department" and "public health commission" in the Blue Pages of Government Listings in telephone books. Information about cancer clusters is also available from other sources. The National Cancer Institute's (NCI) Cancer Mortality Maps & Graphs Web site provides interactive maps, graphs, text, tables, and figures showing geographic patterns and time trends of cancer death rates for the time period of 1950 to 1994 for more than 40 cancers. It also provides interactive mortality charts and graphs, customizable mortality maps, and links to related domestic and international Web sites, including a link to the online publication of NCI's Atlas of Cancer Mortality in the United States: 1950-1994. The NCI's Cancer Mortality Maps & Graphs Web site can be accessed at http://cancer.gov/atlasplus/ on the Internet. General information about environment-related diseases and health risks is available from the National Institute of Environmental Health Sciences (NIEHS). The address for the NIEHS is Room B1C02, Building 31, 31 Center Drive MSC 2256, Bethesda, MD 20892. The NIEHS Web site is located at http://www.niehs.nih.gov on the Internet.
| |
|
|
| |
|
External Sources
 | The National Cancer Institute.
|
| Dollinger M, Rosenbaum, EH, Tempero M., et al. Everyone's Guide to Cancer Therapy, Fourth Edition, Kansas City, Missouri: Andrews McMeel Publishing, 2002.
|
|
|
| |
This article was reviewed and updated June 2007.
|
