An investigation of two apparent cancer clusters in one community

Two apparent cancer clusters in a single municipality were brought to public attention within a few years of each other. The clusters occurred first in a high school and later in a general hospital across the street. Analysis of the clusters was based on the comparison of the observed with the expected cancers, where the expectation of cancer occurrence was derived from data in the Ontario Cancer Registry. While the observed/expected ratios were statistically significant for the high school staff and students, these ratios were based on a very small number of cases (n = 6), and represented 4 different cancer types. The data for the hospital did not appear as excess cancer, either across age groups or in different working locations within the hospital. In addition, cancer rates were examined for the town in which the high school and hospital were located. A number of alternative explanations are discussed, as are the issues surrounding the analysis of cancer clusters in the community. We concluded that the two apparent clusters did not require further study.