The US Centers for Disease Control and Prevention (CDC) defines a cancer cluster as a greater than expected number of cancer cases within a group of people who are linked by some factors. They may be linked by geography, by type or place of work, or by some other factor such as time. The idea of a cluster is often that some common factor or exposure may be responsible for the cancer. However, this is often difficult to prove, for a number of reasons: (1) Cancer is a common disease; (2) All cancers are not the same, and their causes are often different as well; (3) There may be too few cases to achieve a reasonable level of statistical certainty; and (4) Many cancers have multiple causes.
When trying to determine whether a group of cancers is due to chance alone, or is a true “cluster,” some of the questions that should be asked include: (a) Is this the same type of cancer, or is it many different types?; (b) Are the cancers occurring in people who typically get this kind of cancer, or are these cancers happening in people who don’t typically get this type of cancer?; (c) Are the cancers occurring in people who are known to have a specific exposure?; (d) If an exposure is suspected of being related to a cancer, is there a reasonable period of time between when the exposure happened and when the cancer happened (because chemicals that are known to cause cancer typically take several years to several decades between when the exposure happens and when the cancer is finally apparent)?; and (e) Are the cancers common cancers, or are they very rare types of cancers?