Why is it difficult to prove the causes of the cancer cluster?

I often, the title can leave you confused. Like the April 30 Washington Post story about 18 people, three of whom are friends, all concentrated in two locations in North Carolina and Alabama, who were diagnosed with a rare form of eye cancer, melanoma of the eye. Those involved are convinced that their cancers must have a common link, but the investigation failed to find a specific cause, or a source linking the cases. The news left many stunned. “It’s hard to believe there isn’t a common thread here,” says Pat Cutham, North Carolina county commissioner.

It may seem like a simple case of cause and effect when people who live in the same area, work in the same workplace, or play on the same sports team develop cancer, especially if it’s the same type of cancer. Some might assume that pollution is to blame. Others may suspect that there are toxins in the floor or on the walls. But proving a connection is often difficult, if not impossible. In fact, very few of the more than 1,000 suspected cancer clusters reported to federal and state health officials each year have been confirmed as cancer clusters, identified by the U.S. Centers for Disease Control and Prevention (CDC). and the National Cancer Institute (NCI). as “a greater than expected number of cancer cases occurring within a group of people in a specified geographic area over a period of time”.

Define groups: it’s complicated

One example illustrates the confusion around the lingering questions that arise when seemingly obvious connections cannot be confirmed. Last summer, world champion Darren Doulton became the fourth Philadelphia Phillies player to die of glioblastoma, an aggressive form of brain cancer. Many people, especially fellow former baseball players, wonder what appears to be a clear link between brain cancer and baseball, or at least between brain cancer and older versions of artificial turf used on stadiums like Veterans Stadium, which Phillies called Home. 1971 to 2003. But the researchers suspect the mass was likely a coincidence, saying their research did not reveal enough evidence to support the belief that the cancers resulted from a single common source. They added that deaths are proportional to the patterns of cancer that occur in the general population:

While the research findings are frustrating, they are not unusual in the world of cancer groups. In some cases, clusters of cancers are normal because the disease is so common, perhaps more common than many think. In the United States, one in three people will develop cancer in their lifetime. Think of a pool stick that breaks a bunch of balls in a game of pool. Some may land in the farthest corners of the table, while others clump together. Some experts, like neighbors or co-workers with cancer, say it’s completely random.

Cancer is a complex disease, divided into more than 100 different types, each with their own risk factors, symptoms, and causes. The disease usually takes years or even decades to develop. Another factor complicating the search for cluster tests: People don’t always stay in the same place.

“The reasons for the difficulty of establishing cancer clusters are many and include the possibility of random chance, the duration of exposure, and the possibility of different people entering and leaving the area or being exposed to different amounts.” – Pamela Creely, MD, Director of Medical Oncology, Cancer Treatment Centers of ^America® (CTCA)

That’s why there are so many cancer groups that researchers can confirm their presence in the workplace, rather than in residential communities, because it’s often easier to test for, says Asha Karibut, MD, an oncologist and hematologist at our hospital. in Tulsa. When co-workers who have inhaled the same fumes, drank from the same tap, or handled the same substance over time, the source is confined to a smaller area and easier to verify. The ‘over time’ aspect is another key, both for the way the cancer forms and for confirming the association between groups. Environment-related cancers occur most often when people (in the case of cancer groups, the same group of people) are exposed to high and repeated doses of carcinogens. For example, researchers who studied cancer clusters in the 1960s discovered a link between malignant mesothelioma and asbestos, a fibrous mineral common in manufacturing, industry and consumer products. Then, in the early 1970s, when workers at the same chemical plant were diagnosed with liver hemangiosarcoma, researchers determined that the rare cancer resulted from workers’ exposure to vinyl chloride, which you now know is a major risk factor for your disease. .

Irrelevant factors overshadow the picture

Other factors, such as smoking, obesity, and age, can make testing for cancer groups more difficult. “Variables such as smoking or obesity can contribute to the development of a range of cancers when, in fact, they are not present,” Dr. Creeley says. Not all people exposed to a carcinogen will develop cancer (some people are more or less likely based on their genes) while others will develop the same cancer, even if they are not exposed to high-risk toxins. Even when carcinogenic agents are found in only one neighborhood, some residents may be exposed to this substance while others may not.

For example, not all of the former Velez players who died of brain cancer have played together at the Veterans Stadium or stayed with the team for the same amount of time. The Philadelphia Eagles also played and practiced on the field, but the National Football League has not reported an unusual incidence of brain cancer among its players. “It is often difficult to scientifically prove a cancer population exists,” says Anthony Berry, MD, chief of the CTCA division of ambulatory medicine. ^® . “It is very likely that the cancer cluster is there. You can’t prove it.”

To confirm a true group of cancer, certain criteria must be met:

• A greater than expected number of cancer cases must occur in a given population.
• Cancer cases must include the same type of cancer or different types of cancer with the same cause.
• All cancers must be primary cancers, which means that the disease started in the same area of ​​the body, not in the organs to which it may have spread.

Based on a set of specific steps health officials take to investigate a cancer group, they collect information, including:

• The geographical area in which the cases occurred
• Number of issues involved
• The type of cancer affected
• Demographic information about people affected, such as age, gender, ethnicity, and occupation.

Officials sometimes decide that no further investigation is needed, such as in cases where cancer has:

• They are found among blood-related family members, especially if cancers are known to have a strong genetic link.
• There are different types that are not caused by the same factors.
• They are of the same type, but only occur in a small number of people, especially those at high risk of developing this type of cancer.
• They occur between people who did not live in the same place at the time the exposure occurred.

If more research is needed, officials compare the information collected with similar data from cancer censuses and registries, such as a group of the same age and sex. The picture becomes even more confusing when you take into account the personal decisions that officials make when determining key elements of a group, such as its geographic boundaries and who might be affected. This means that in some cases, two health departments can come to different conclusions about whether a cancerous mass is real or a coincidence.

In 2015, when a suspected group of brain cancer was reported in the small town of Pelius Creek, the North Carolina Central Cancer Registry investigated cases of cancer in counties with coal ash storage facilities. Most city residents rely on wells for drinking water, and many blame a storage pond that Duke Energy uses to collect waste from the nearby coal-fired power plant. However, the investigation could not confirm that the incidence of cancer in Pelius Creek was higher than in other similar areas of the state. But critics say the study unfairly used county-wide numbers, which may not accurately reflect, for comparison purposes, the number of cancer patients living near a coal ash pond. In December 2017, environmental and civil rights groups sued Duke, alleging that it polluted the community’s water supply. The case is still in court.

Science is limited, but it gets better

Experts say those frustrated by the lack of conclusions about cancer populations should also consider the limitations of the science. Investigators often lack the conclusive evidence necessary to make the connection. For example, if many believe that asbestos is the culprit, but all traces of the mineral have disappeared over time, researchers have no evidence to rely on. Or, if some suspect that your well water is contaminated with a carcinogen, researchers may not be able to tell who is drinking the water and who is not. It doesn’t help that most cluster investigations involve a small number of cases, which can make it difficult for researchers to draw statistically significant conclusions. “Many reported cancer cohorts simply do not include enough cases to allow researchers to establish that there is a statistical difference from the expected number of cases,” Dr. Perry says.

But experts say the technology is helping to clarify at least part of the picture. In recent years, scientists have developed more sensitive tools for measuring exposure, and the Centers for Disease Control (CDC) and the National Cancer Institute (NCI) now use software designed to analyze geographic cancer data to determine whether groups are statistically significant. National Cancer Registries, which help establish a baseline of cancer incidence and detect abnormalities more easily, have become another important resource for researchers in the collective research. “There are still many challenges in testing a cohort for cancer, but the science continues to advance, and epidemiological studies continue to improve,” Dr. Creely says.