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.
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.