Generally speaking, a cancer cluster is defined as a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time. This is done by looking at the number and rates of cancer cases that are occurring (observed) and comparing it to the number and rates that would be expected in the same population. An apparent excess of cancer may occur due to:
- Chance
- Miscalculation of the expected number of cancer cases (e.g., not considering a risk factor within the population at risk)
- Differences in the way the cancer was identified between observed cases and expected cases
- Known causes of cancer (e.g., smoking)
- Unknown cause(s) of cancer. It is important to note that the term "cluster" tends to imply different things to different people, which leads to a large amount of confusion. For example:
- The term is most often used when the increase in cancer incidence is unexplained or contested. In this sense, if an explanation such as an excess of elderly residents in a community were documented, the increase may no longer be considered a "cluster".
- Others label cancer increases as clusters only when an environmental cause is known or suspected. This definition may or may not conflict with the one described above.
- Some commentators assume that the term can only be applied to small areas such as a county or part of a county, regardless of its cause. In this case, larger regions (for example "Southern California" or "The Panhandle of Florida") are not considered clusters even if they experience a documented excess of cancer cases.
For each of the above definitions, the appropriate methods for statistical analysis and documentation of the cluster are very different. Interestingly, other diseases and conditions such as preterm birth and asthma are rarely labeled "clusters" even though the above situations as applied to them occur frequently.
Most frequently, questions from citizens about possible clusters are discussed with local or state health department staff and are addressed satisfactorily with education about cancers and cancer risk factors. However, some situations will require further evaluation to understand what may be going on. The follow-up evaluation may consist of a routine review of data in the cancer registry, or could require extensive investigation that may take years to complete. Because cancers are complex diseases, however, in many instances, no clear cause of a true cluster is ever found.
