What is cancer?
“Cancer” is one term that is used to refer to over one hundred different diseases. These diseases have in common an uncontrolled multiplication and growth of abnormal cells and the ability to spread to body parts that are distant from the original site. Cancer is a very commonly occurring group of diseases. Nationwide, it strikes three out of four families, is diagnosed in one out of three people, and causes one out of five deaths.
Where do cancers form?
Cancers in different sites and cell types represent entirely different diseases. Cancers can start in many different body sites, and even in different tissues within a given site. Different cancers also spread to different body parts. For example, cancer can begin in the mouth and is most likely to spread to the lungs, while cancer starting in the breast is more likely to go to a bone, and cancer originating in the stomach may progress to the liver.
What causes cancers to form?
Different cancers also have different risk factors which promote their development. For example, cigarette smoking causes lung cancer, overexposure to the sun has been shown to be associated with the development of skin cancer, and lack of dietary fiber may be related to colorectal cancer. Other risk factors that have been identified include alcohol abuse, family history of cancer, age, sex, race, and some specific occupational exposures. Smoking, alcohol abuse, and poor diet account for 80% of the cancer deaths that occur.
The degree to which environmental pollution causes cancer is not precisely known, but most experts agree that less than five percent of cancers are caused by pollution. Given that specific exposures are linked to specific types of cancer, if an environmental exposure caused cancer, its effects would be manifest in the occurrence of cases of the same type of cancer.
How quickly does cancer develop?
Cancers also differ with respect to latency, or the time between exposure to one or more cancer-causing agents and the development of cancer. Generally speaking, however, cancers commonly take 10 to 30 years or more to develop to the point of being detectable. When looking for the cause of cancer, one must consider exposures that took place at least ten years before the cancer was diagnosed.
What is the Cancer Cluster?
A cancer cluster is defined as a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time.
To be a cancer cluster, a group of cancer cases must meet the following criteria. Until all of these parameters are met, the group of cancer cases is often referred to as a suspected cancer cluster.
- A greater than expected number:
A greater than expected number is when the observed number of cases is higher than one would typically observe in a similar setting (in a group with similar population, age, race, or gender). This may involve comparison with rates for comparable groups of people over a much larger geographic area – e.g., an entire state.
- Of cancer cases:
All of the cases must involve the same type of cancer, or types of cancer scientifically proven to have the same cause.
- That occurs within a group of people:
The population in which the cancers are occurring is carefully defined by factors such as race/ethnicity, age, and gender, for purposes of calculating cancer rates.
- In a geographic area:
Both the number of cancer cases included in the cluster and calculation of the expected number of cases can depend on how we define the geographic area where the cluster occurred. The boundaries must be defined carefully. It is possible to “create” or “obscure” a cluster by selection of a specific area.
- Over a period of time:
The number of cases included in the cluster – and calculation of the expected number of cases – will depend on how we define the time period over which the cases occurred.
How much cancer is too much?
Unfortunately, one out of every three Americans will develop some form of cancer during his or her lifetime. Although it sometimes seems as if cancer is occurring now with greater frequency, it is important to remember that cancers are primarily diseases of old age and that the American population now includes a higher proportion of elderly persons that ever before. Secondly, at one time the majority of people died from heart disease, infectious disease, or injuries. As we have made progress against these other health problems, more people are living long enough to develop cancer. Finally, much of the stigma formerly associated with having cancer has disappeared, and cancer patients are more likely to talk about their illnesses than previously.
When epidemiologists receive reports of a group of cancers occurring among persons who live close together, they must decide whether a cluster actually exists. Cancer cases involved in a cluster would:
- Be unusually high in number
- Be of a specific body site
- Occur close together with respect to space and time.
Defined clusters may be due to some identifiable factor or may occur simply by chance. This judgment is usually made on the basis of knowledge of disease patterns and probability statistics. The vast majority of apparent cancer clusters are chance events and not due to some identifiable common cause. In order to be linked to a common cause, it must be biologically possible for the substance not only to have been released into the environment, but also to have come into contact with persons in the surrounding area through a means that would present a risk and in a sufficient dose to cause illness.
The development of cancer is a complex and not fully understood process. Given the differences between the various types of cancer and the complexity of the development of cancer, when considering cancer causation, one must be careful not to think that all cancers are the same. Personal lifestyle factors account for most cases of cancer and are much more significant risk factors than are environmental exposures. Not all cancer can be prevented, but the best way to minimize your risk is to avoid factors known to be related to cancer and to participate in routine screening programs in order to catch cancer in early stages, which increases the chances of a favorable outcome. Examples of these examinations include rectal examinations, Pap tests, breast self-examinations, and mammography. Further, the EPA recommends all homes be tested for radon, regardless of geographic location or the zone designation of the county in which they are located.
More information on Cancer Clusters
The American Cancer Society published a very useful article called Understanding Cancer Clusters.
The CDC’s Agency for Toxic Substances and Disease Registry provides information that discusses the relationship between environmental risks and cancer. This information answers questions about how cancer may be related to environmental risks.