You asked for the scientific or medical definition of "cluster" as it is used in reference to cancer.

According to the National Cancer Institute (NCI), a disease 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. Clusters of various diseases have concerned scientists for centuries, according to NCI. Recent examples of disease clusters are the outbreak of Legionnaire's disease in the 1970s, and periodic outbreaks of food poisoning caused by food contaminated with bacteria.

A cancer cluster is a real or meaningful number of the same type of cancer occurring during a short period of time among people who may live or work in the same area. Cancer clusters may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related cancer.

Generally, the study and investigation of a possible cancer involves several questions: (1) are they all the same type of cancer; (2) during what period of time did these cancers appear; (3) how old are the people with cancer; and (4) is it a rare cancer. Next, the cases reported must actually be confirmed as cancer and then a determination made if the number of cases is higher than the number of expected cases.

JK: eh