How to Investigate a Cancer Cluster

An unusual number of local childhood leukemia cases in the 1960s and ’70s raised concerns for residents of Woburn, Massachusetts, that a potential environmental exposure was making children sick. An investigation by the Massachusetts Department of Health and the CDC confirmed their suspicions, finding that this cluster of cases was likely caused by contaminated well water.

A cancer cluster is an occurrence of a greater than expected number of cases of the same or related cancers among a group of people in a defined geographic area over a specific time period. After thorough investigation, like in the one in Woburn, some cancer clusters can be traced to an environmental exposure—but conclusively identifying a cause is extremely rare.  

In this Q&A adapted from the December 10 episode of Public Health On Call, Megan Latshaw, PhD, MHS, an associate teaching professor in Environmental Health and Engineering, and Suzanne Condon, the former associate commissioner of the Massachusetts Department of Health, explain how cancer clusters are studied and why these investigations are so important, even when outcomes are inconclusive. 

How do health departments learn about potential cancer clusters? 

Suzanne Condon: There are different ways that people approach a state health department and ask for help in investigating these types of patterns of cancer.  Sometimes it can come from a community member, a parent, for example, of a child with cancer. Other times it might come from a legislator, the media, or an academic partner.  

The childhood leukemia cluster in Woburn essentially came to us as a result of parents talking to their pediatric oncologist, and saying something’s wrong, and the oncologist seeing people who lived in the same neighborhood that were his patients. 

What’s the first step in investigating a cancer cluster? 

Megan Latshaw: One of the most basic things that a health department will do is calculate a standardized incidence ratio, which is the number of observed cases divided by what you would have expected to see in the population. States typically have a cancer registry that collects data on cancer. If you’re observing more cases than you would expect to see, then there might be something going on.  

Then, you’ll look at whether something is happening in the community that might be causing excess rates of cancer: What are the exposures in that community? Do they have high radon levels? Do they live near a hazardous waste site? A lot of times the community has theories and may even know what their own exposures are. An important part of a cancer cluster investigation is involving the community from the very beginning. 

What is needed to research unusual patterns of cancer? 

ML: These investigations take money, resources, and a lot of people power—epidemiologists, toxicologists, environmental scientists, and other experts. If you don’t have those people or resources, you’re going to miss things and not be able to investigate. 

SC: The community-driven resource is critical. One of the most important things I learned was in a study we did of young men with angiosarcoma [a rare cancer that develops in the inner lining of blood vessels and lymph vessels] in the community of Ashland, Massachusetts. I went to the community and explained that the numbers looked high, but that the kind of study necessary would require us to go back and identify people who lived here and went to high school here decades ago.  

A woman stood up in the back of the room and said, “Suzanne, that won’t be a problem here. We know where every student that went to school here lives. We keep track of those things.” There was also a person in the room from the local senator’s office who committed to finding the resources to pay for that study. At the end of the day, we did see a connection between exposure to a particular hazardous waste site and higher rates of rare cancer in that community. 

Tell us about the cancer cluster investigation in Woburn. 

SC: We began getting reports from our clinical partners about what they were seeing, which appeared to be an unusually high number of children with leukemia. Upon investigation, we found a significant number of children who lived in the northeast section of Woburn who had leukemia. [Twelve cases were identified between 1969 and 1979; an additional nine were identified through 1986.] 

Community members were focused on the drinking water, which was found to be contaminated with industrial cleaning solvents. Early in our investigation, the two wells that were primarily responsible for contaminating the water were taken offline, and the drinking water quality improved significantly. However, exposures had already taken place.  

One of the interesting pieces about Woburn was that children exposed after birth did not show an increased risk of developing childhood leukemia. It was their mothers’ exposures during pregnancy—in utero exposure—that really was the smoking gun. 

When we did trend analysis, we saw increased risk with increasing exposure—as much as eight times greater risk—for those children whose mothers consumed that water during pregnancy. 

What should investigators keep in mind when looking at potential cancer clusters? 

ML: One of the key questions is, what cancers are increasing? And do they have similar causes? Because cancer is not a single disease. It is many, many diseases that have different mechanisms and causes.  

I do want to point out that the Woburn case is unusual because most cancer cluster investigations don’t end up yielding conclusive results, which can be really difficult for communities to hear. In part, it’s because cancer is such a complicated disease; sometimes cancers can take decades to develop, making it very difficult to go back and identify exposures.  

Why is it important to investigate cancer clusters, if they’re rarely confirmed? 

ML: Health departments need to be doing these investigations even if we don’t always find the results because when we do find results, they lead to interventions that save lives. Imagine how many more people in Woburn would have been poisoned if the state health department had not been able to do that investigation. And as our tools get better, I would like to be able to catch these earlier. I have a lot of hope that we can even train AI to help us detect unusual patterns of cancer before doctors see an increase in cases in their communities.