CDC issues New Guidelines on Analyzing Cancer Clusters With Environmental Cause

New guidelines step away from heavy reliance on statistical significance and focuses more on rapid detection of cancer clusters

As health officials regularly investigate reports of cancer clusters throughout the U.S., a series of updated guidelines have been issued to help health departments conduct evaluations and determine whether environmental hazards or toxic pollutants may be the cause.

The U.S. Centers for Disease Control and Prevention (CDC) and the Agency for Toxic Substances and Disease Registry (ATSDR) issued updated guidelines for examining unusual patterns of cancer and environmental concerns on December 8, recommending frequent proactive reviews of cancer registry data to identify true cancer clusters more rapidly.

Extensive scientific literature was reviewed to form the new guidelines, and the agencies gathered feedback from focus groups, meetings, and interviews with experts, community, state, tribal, and local communities.

The new recommendations place greater emphasis on engaging and communicating with the community, and updates draft guidelines first issued by the CDC in June 2022. Until this latest version, more importance was placed on the statistical significance of cancer patterns, instead of other potential factors.

The updated guidance replaces the steps in the 2013 guidelines with new investigative phases focused on:

  • Addressing environmental concerns more broadly
  • De-emphasizing statistical significance as the primary criteria for clarifying unusual patterns as clusters.

Defining Cancer Clusters

Cancer clusters occur when an unusual number of specific cancer cases occur in a limited geographical area, often suggesting there may be environmental factors in that area increasing the risk of cancer for those exposed.

Rates of cancer among cluster patients are much higher than seen in the general population. And previous cancer cluster investigations relied heavily on detecting a statistically significant increase of a certain type of cancer when compared to other areas.

For example, in 2020, a mother from North Carolina raised more than $100,000 to pay Duke University to investigate a suspected thyroid cancer cluster linked to coal ash basins near Mooresville.

The initiative began after her teenage daughter and several other people on the same street were diagnosed with papillary thyroid cancer. Later, the local health department found double the number of expected thyroid cancer cases, prompting suspicions of a cancer cluster from coal ash.

Guidelines Seek Rapid Detection of Cancer Clusters

The new guidance focuses on how local health departments, like Mooresville, and cancer registries can determine more quickly if they have true clusters because in some cases the patterns may be chance or the result of other factors.

Some factors can include differences in cancer screening practices, differing access to healthcare due to socioeconomic factors, genetic susceptibility to particular cancers, occupational exposures, and environmental exposures.

The causes of many cancers are often unknown but, in some cases, the cancer risks have been tracked back to environmental or occupational exposures to cancer-causing substances.

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The new guidelines include a decision-making tool that can be used by health departments to determine the need to further assess and investigate potential clusters. The tool contains criteria for assessing the patterns of environmental risk factors.

The tool focuses on various factors to determine if a pattern is indeed a true cluster:

  • Greater than expected number of cases in an area
  • Are the same type or etiologically related
  • Occur within a group of people
  • Occur within a specific geographic area
  • Occur over a defined period of time

The CDC continues to identify and develop tools and templates to support public health partners to better conduct cancer cluster investigations and protect the health of the public.


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