When a large number of people in a community come down with cancer, doctors and researchers try to find out if there's a root cause. For more information about cancer clusters, explore the questions and answers below.

Q: What is a cancer cluster?

A: It is a greater than expected number of cases of a particular kind of cancer in a group of people in one area over a period of time. A true cluster usually involves: one type of cancer, a rare type of cancer, or a type of cancer that strikes people at an age when they usually don't get that kind of malignancy.

Q: What kinds of cancer usually represent a true cluster?

A: Childhood leukemia is most often investigated. In Vermont, health authorities are currently trying to find the cause of several leukemia cases in Clarendon.

Q: What causes most clusters?

A: Experts say most seem to be a matter of chance. The Centers for Disease Control and Prevention has never identified the cause of a single cancer cluster.

Q: Why do there seem to be so many of them?

A: One in four Americans die of cancer, so it is no surprise the disease often strikes several people in the same neighborhood, the same block or even the same family.

Q: Don't chemicals and other environmental toxins cause cancer clusters?

A: They may cause some, but disease investigators have never been able to prove it. Often the cancer appears years later, long after the pollutant that possibly triggered it has disappeared.

Also, pollutants in the air or water probably account for just 1 percent to 2 percent of cancers, according to the American Cancer Society.

Q: What about famous clusters I've heard about, like the one in the movie, "Erin Brockovich?"

A: Lawyers sometimes succeed with lawsuits claiming that pollution causes cancer clusters, but that does not mean the link is proven in a scientific way.

Q: What about the high number of breast cancer rates in places like Marin County, Calif., and parts of Long Island, N.Y.?

A: Disease investigators say that the cancer rates for the Marin County women are not high considering the demographics of the women who live there: They are older, many had children later in life or not at all, and they had access to hormone supplements -- all factors that raise the risk of breast cancer.

On Long Island, studies have found no link between breast cancer and power lines or pollutants.