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Cancer Clusters
A cluster is 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.
Epidemiologists have identified certain conditions that should be considered before reporting a suspected cancer cluster: a suspected cancer cluster is more likely to be a true cluster if it involves a large number of cases of one type of cancer, rather than several different types; a rare type of cancer, rather than common types; or a number of a certain type of cancer cases in age groups not usually affected by that type of cancer.
Local and state health departments are responsible for conducting cancer cluster studies and are the agencies to which a suspected cluster should be reported. The Centers for Disease Control and Prevention and other Federal agencies, which prior to 1990 were responsible for cluster studies, are now called in only for special situations.
More information is available from the following: 
The National Cancer Institute
A primary fact sheet on cancer clusters and risk factors, plus additional cancer-related links.National Cancer Institute Research on Childhood Cancers
A fact sheet on cancer in children, including information on cancer clusters.National Institute of Environmental Health Sciences
reprint of an article from the journal "Environmental Health Perspectives" which deals with childhood cancer; includes information about cancer clusters among children.Centers for Disease Control (CDC)
Guidelines for Investigating Clusters of Health Events
