Cancer Cluster Information

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. Cancers, as a group, are very common. Cancers are the second leading cause of death in the United States and Virginia, exceeded only by diseases of the heart and circulatory system. According to the American Cancer Society, one of every two men and one of every three women will be diagnosed with some form of cancer at some time in their lives.

What causes cancers to form?

Different cancers 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 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. In  children, the time from exposure to cancer development is shorter, 1-10 years. 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 a Cancer Cluster?

A cancer cluster is defined as an “a greater than expected number of the same or etiologically related cancer cases that occurs within a group of people in a geographic area over a defined 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: When the number of observed cases is greater than typically observed in a similar setting.
• Of the same or etiologically related cancer cases: Cases are of the same type, are within a family of tumors (e.g., Ewing’s family of tumors), or have a known or suggested link to the same specific environmental or chemical exposures. It is possible to consider multiple cancer types when such a known exposure (e.g., radiation or a specific chemical) is linked to more than one cancer type or when more than one contaminant or exposure type has been identified.
• Within a group of people: The population in which the cancer cases are occurring is defined by its demographic factors (e.g., race, ethnicity, age, and sex).
• In a geographic area: The geographic area may be based upon pre-existing geopolitical boundaries (e.g., census tract, county, or ZIP code/ZIP code tabulation area). It may be defined according to the nature and extent of potential exposures that may cross multiple or partial boundaries. These geographic boundaries are used to determine the number of cancer cases as they relate to the total population in this predefined area. It is possible to create or obscure a cluster inadvertently by modifying the area of interest.
• Over a period of time: The time frame used to establish the beginning and end dates for analysis. The time period chosen for analysis will affect both the total cases observed and the calculation of the expected incidence of cancer in the population.

Are cancer clusters common?

No, real cancer clusters where a cause can be identified are extremely rare. There have been no proven cancer clusters in Virginia, and only a few around the United States. In a scientific review of over 500 cancer cluster investigations done over 20 years, only about 1 in 8 found a true increase in cancer rates, and in only one case was a clear cause for the increase found.

There are five people on my street with cancer; is this a meaningful cluster?

Not necessarily. Cancer is very common, especially among older people, and each type of cancer may have a different cause. By chance, a group of people on your block could have a number of different cancers, but it may not be a meaningful cluster.

How are cancer clusters investigated?

When a suspected cancer cluster is reported, staff from the Virginia Cancer Registry will first review the initial information provided to confirm the correct type of cancer, where the cancer started within the patient, and when it was diagnosed. This step also includes reviewing doctors’ notes and test results to ensure accuracy. Once geography of interest, population, and time frame are defined, an epidemiologist compares the cancer rates to Virginia or another comparable geography. The VCR also collaborates with the Environmental Public Health Tracking (EPHT) Program and Public Health Toxicology Program to address environmental concerns when needed.

Based on this initial investigation, most suspected clusters can be determined not to be true clusters, and no further investigation is needed. Only when data point to an indication of a cluster or unusual pattern of cancer would an in-depth investigation may occur, to include technical assistance from the CDC and/or the Agency for Toxic Substances and Disease Registry (ATSDR). According to guidelines from the CDC, some factors that do not support the need for further investigation include:

• Cancer cases within family members who are blood relatives (especially cancers known to be strongly genetically linked).
• Different types of cancers that are not known to be related to one another.
• A few cases of very common cancers, particularly when the people affected are of the usual age and sex for those cancers.
• Cancers among people who didn’t live in the same area at the time an exposure would have to had taken place to cause that cancer.

What are the possible outcomes of a cluster investigation?

There are three possible outcomes from a cancer cluster investigation:

• In most cases, an investigation will show that the suspected cluster is not a true cancer cluster.
• Less often, an investigation finds a true cancer cluster, but no cause can be found.
• Rarely, an investigation finds a true cancer cluster with a cause or no cause.

Cancer Prevention

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. For more information and resources, visit https://www.vdh.virginia.gov/cancer/.

More information on Cancer Clusters

Centers for Disease Control and Prevention information on cancer clusters and Cancer Clusters FAQ.

National and State Cancer Data and Interactive Map from the National Cancer Institute

National Cancer Institute information on cancer clusters

American Cancer Society information on cancer clusters