A cancer cluster is a geographic area that has an unusually high number of cases of the same cancer during a specific time period. To be considered a cancer cluster, the population must show an increase in a rare type of cancer, an increase in the same type of cancer, or an increase of cancer in a group that is generally at low risk for that type of cancer.
“Breast cancer rarely meets any of these criteria, as it is common and there are many distinct types of breast cancer,” explains Dennis Citrin, MD, PhD, a medical oncologist with Cancer Treatment Centers of America at Midwestern Regional Medical Center in Zion, Ill.
State and local health departments, which collect data on cancer incidence, are usually the first to suspect a cancer cluster. Then scientists step in to do further research, such as obtaining information about the environmental exposures,health, and lifestyle of residents in the cluster area. Residents might also have urine or blood samples taken to look for possible exposure to carcinogens. When a cluster is identified, the health department might ask the Centers for Disease Control and Prevention (CDC) for assistance in identifying the cause of excessive cancer cases in a particular area.
Learning From the Past
In the last 50 years, many cancer clusters have been studied in the United States to try to determine genetics versus environment and breast cancer causes. It was discovered that most are due to exposures on the job, and a few are the result of residential environmental exposures. Two well-known breast cancer clusters thoroughly studied in the 1990s were on Long Island in New York, and Marin County in California, in the San Francisco Bay area.
“The Long Island study was one of the first to show higher breast cancer risk in women who had higher levels of DNA damage in their blood from exposure to polycyclic aromatic hydrocarbons (PAH), products of combustion,” says Julia Brody, PhD, executive director at Silent Spring Institute in Newton, Mass., a research organization dedicated to studying the links between the environment and women’s health. That finding was important because PAH are common in air pollution, car exhaust, charred (heavily grilled) food and foods cooked at high temperatures, and tobacco smoke. These toxins have caused cancer in animal studies. “By analyzing blood samples, Long Island study researchers found that, for women diagnosed before menopause, PAH exposures appeared to be related to increased breast cancer risk.”
The Long Island study also showed that the blood levels of dichloro-diphenyl-dichloroethylene (DDE), a residue from the use of the toxic chemical dichloro-diphenyl-trichloroethane (DDT), measured in older women around the time they were diagnosed with breast cancer, were not associated with increased breast cancer risk. However, a study in California, published in Reproductive Toxicology, found that women who had high levels of DDT in their blood from exposures during girlhood had significantly higher breast cancer risk, Dr. Brody reports. What’s more, that study pointed out the limitations in previous studies that failed to make the DDT–breast cancer connection.
Ultimately, the studies on Long Island and in Marin County showed that the excessive number of breast cancer incidences were mostly due to risk factors, such as genetics, alcohol use, and never having a baby or having a baby later in life. “This means that the cancer was not caused by carcinogens in the environment, but was a result of the commonalities of the population,” Dr. Citrin says.
The Value of Studies on Environmental Factors
Brody says it’s important to do studies when there is an increased incidence of cancer in a geographical area. “When scientists observe an unusual number of cancers in a particular group of people, this can lead to discoveries about the cause,” she says. “By understanding cancer causes, we learn how to prevent it.”
Brody also notes that dozens of chemicals and toxins to which we are exposed every day — in the air and water, or in everyday products such as cleaners,cosmetics, and furniture — can disrupt breast-related hormones or cause breast cancers in animal studies.
Visit the CDC’s interactive Tracking Network Web site to see how your state compares to other states regarding rates of breast cancer and certain environmental factors. Your local or state public health department may also have information on local breast cancer rates, as well as information about cancer-cluster investigations.
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