Definition of Cancer cluster

Cancer cluster: A greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time. The expected number of cancer cases is calculated and compared to the observed number of cancer cases. A cancer cluster is confirmed when the observed/expected ratio is greater than 1.0, and the difference is statistically significant.

What first appears to be a cancer cluster may not be one. Scrutiny of the situation may show that the number of cancer cases is within the expected range for the population.

Cancer cases are most likely to represent a cancer cluster if they involve: (1) one type of cancer, (2) a rare type of cancer, or (3) a type of cancer in a group not usually affected by that cancer. For example, a cancer normally seen in adults occurs in children.

Last Editorial Review: 6/14/2012

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