The National Cancer Institute (NCI) defines a disease cluster as "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."
The Centers for Disease Control and Prevention defines a cluster investigation as "a review of an unusual number, real or perceived, of health events (for example, reports of cancer) grouped together in time and location." If possible, the investigation looks for causes.
Some causal relationships, such as the link between smoking and lung cancer, have been well established. But while air, soil and water quality remain sources of great public concern, "few community-level environmental exposures have been well studied," according to the CDC's National Environmental Public Health Tracking Network.
It was the discovery of an unusually high number of cases of a rare type of pneumonia among homosexual men in the early 1980s that led eventually to identification of the human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS).
Cancer, the uncontrolled growth and spread of abnormal cells anywhere in the body, is an umbrella term for many different but related diseases, each with certain risk factors, known or suspected, associated with it.
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Asbestos
One of the best known examples of a cancer cluster emerged in the 1960s, when doctors reported many cases of mesothelioma, a rare cancer of the lining of the chest and abdomen. Researchers found exposure to asbestos the likely shared risk factor.
A cancer cluster may first come to light when citizens report that several relatives, friends, neighbors or co-workers were diagnosed with the same or related cancers. Epidemiologists look especially for a large number of cases of one type of cancer, rather than several different types, or numbers of an otherwise rare type of cancer, or a higher than expected number of cases in an age group not usually hit by that type.
In Sierra Vista, Ariz., population 40,000, the CDC launched an investigation in 2004 after 12 children were diagnosed with leukemia -- three times the statistically probable number. Residents raised questions about the nearby location of two military bases and a jet fuel pipeline that ran beneath the town.
In Fallon, Nev., 16 children were diagnosed with leukemia between 1997 and 2003. The CDC said it could find no links between the illnesses and environmental contaminants.
"However, biomonitoring tests conducted by CDC found residents in Fallon had elevated levels of arsenic and tungsten in their urine," according to a watchdog group, Families Against Cancer and Toxins. "A tracking network that collects data on environmental exposures and health effects could have helped discover the cluster earlier and aided health officials in their investigation."
Landfills, arsenic, lead or mercury?
Residents of Lassen County, Calif., have raised concerns about open burning and detonations at a downwind ammunition depot. People in Daly City, Calif., have wondered whether soil contaminated by toxins was responsible for a rash of genetic defects and illnesses.
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In Connecticut, researchers looked at concerns about a middle school built over a former landfill, and a high number of diagnosed brain cancers among employees of a jet-engine manufacturing company. In Florida, arsenic, lead, mercury and other substances from contaminated Superfund sites have been fingered as suspects in high rates of cancer, birth defects and babies with low birth rates. In Massachusetts, the suspected culprit for a cluster of Lou Gehrig's disease and multiple sclerosis is toxic waste from local industries and agricultural pesticides and herbicides.
Some of those investigations have gone on for years. But often, the CDC reports, "without adequate tracking of these substances and human health, it is difficult to determine whether the environment is linked to these illnesses."
Tracking cancer
As part of the national "war on cancer" begun in the 1970s, health agencies in each state were directed to compile information about cancer rates and study patterns and trends. In North Dakota, such "cancer tracking" is done by a cancer registry office within the state Department of Health.
In recent years, citizens have brought a number of concerns to the office, "but nothing has panned out," said Marlys Knell, the registry's director.
The registry office and others in the health department have looked at Grand Forks County data from 1997 to 2006 and have found "nothing of concern" regarding cancer rates. They say they will examine cancer incidence and mortality data for 2007 as soon as it is available.
Reach Haga at (701) 780-1102; (800) 477-6572, ext. 102; or send e-mail to chaga@gfherald.com .
