Reported disease clusters of any kind, including suspected cancer clusters, are investigated by epidemiologists (scientists who study the frequency and distribution of diseases in populations). Epidemiologists use their knowledge of diseases, environmental science, lifestyle factors, and biostatistics to try to determine whether a suspected cluster represents a true excess of cancer cases.
Epidemiologists have identified certain circumstances that may lead them to suspect a potential common source or mechanism of carcinogenesis. A suspected cancer cluster is more likely to be a true cluster if it involves:
A large number of cases of one type of cancer, rather than several different types;
A rare type of cancer, rather than common types, or
An increased number of cases of a certain type of cancer in an age group not usually affected by that type of cancer.
Before epidemiologists can accurately assess a suspected cancer cluster, they must determine whether the type of cancer involved is a primary cancer or a cancer that is the result of metastasis (spread from another organ). This is important because scientists consider only the primary cancer when they investigate a cancer cluster.
Epidemiologists also try to establish whether the suspected exposure has the potential to cause the reported cancer, based on what is known about that cancer's likely causes and what is known about the carcinogenic potential of the chemical and the type of exposure. Scientists use various statistical methods to determine whether the reported excess of cases is really a larger number than would normally be expected to occur.
For a variety of reasons, most reported cancer clusters are not shown to be true clusters. Many reported clusters do not include enough cases for epidemiologists to arrive at any conclusions. Sometimes, even when a suspected cluster has enough cases for study, a true statistical excess cannot be demonstrated. Other times, epidemiologists find a true excess of cases, but they cannot find an explanation for it.