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Frequently Asked Questions

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Several people in my neighborhood have been diagnosed with cancer. Is this unusual?
What is a cancer cluster?
What are some common misconceptions about cancer clusters?
Why are cancer rates higher in some areas than others?
How do individual characteristics and behaviors affect cancer rates in my neighborhood?
How can so many cancer cases in my neighborhood be due to chance?
Does the environment cause cancer?
Why haven't investigations of cancer clusters been useful in identifying environmental causes of cancers?
If there are a lot of pets in my neighborhood with cancer does it mean there is a problem with the environment?
Who should I contact with my questions about cancers in my community?
How will my concern be addressed?

Several people in my neighborhood have been diagnosed with cancer. Is this unusual?

No, this situation occurs more often than you might expect. People often wonder if there are "too many cancers" in their neighborhoods, but most of the time it turns out that the number is about what we would predict. Here are some reasons why there may be quite a few people living in your neighborhood who have been diagnosed with a cancer:

  • Cancers are very common. In California, about 45% of all men and 41% of all women will develop a cancer sometime during their lives (www.ccrcal.org/questions.html).  Therefore, you will find people who have been diagnosed with a cancer in just about every neighborhood in the state. Cancers are most common in neighborhoods with lots of older residents because cancer risk increases with age.
  • Cancer is not one disease. There are many types of cancers. Different cancers have different risk factors, treatments and outcomes. So, even though all cancers involve out-of-control growth of cells, the different cancers are really different diseases. So for example, if your neighborhood has three people with three different cancers (such as lung cancer, breast cancer, and liver cancer), those three people actually have three different diseases - even though they all are called "cancer." These three cancers have very different causes, so there would be no reason to think that one common factor in the neighborhood would be to blame.
  • Cancer rates often vary from year to year and from place to place by chance. The number of cancer cases will never be exactly the same in each neighborhood. In some places the number will be higher than average and some places it will be lower than average for no reason other than chance. Your neighborhood could just happen to have a higher than average number of people with new cancers just by chance.
  • People diagnosed with cancer are living longer. Currently, over half of all persons diagnosed with a cancer will be alive for five years or more after their diagnosis. Therefore, the chances are better than ever that there are a number of cancer survivors living in your neighborhood.
What is a cancer cluster?

A cancer "cluster" is a situation in which there are more cancer cases in a group of people, in a given location, or in a time period than would be expected based upon usual patterns. A cancer "cluster" usually refers to an excess of one particular type of cancer. A grouping of several different types of cancers is not usually considered to be a "cluster."

Public health officials and researchers who respond to questions about clusters from the public think of "clusters" in two distinct ways. The first is a suspected cluster (a concern has been reported, but not yet evaluated). The second is a confirmed cluster (where officials have confirmed that the number of cases is in fact higher than expected). Public health officials nationwide receive hundreds of inquiries every year about suspected clusters. However, after an evaluation, only a small fraction of these suspected clusters are actually confirmed.

Many people assume that if a cluster is statistically confirmed it means there is something wrong with the neighborhood environment (such as pollution of the air, water, or soil). This is not necessarily the case. Usually the reasons cancers "cluster" in residential neighborhoods have little to do with the physical environment. In most cases, cancers "cluster" in neighborhoods because residents have similar cancer risk factors (such as smoking or being older), or by chance.

What are some common misconceptions about cancer clusters?

Misconception #1: Cancer "clusters" are rare.
Fact: The California cancer registry tracks more than 80 types of cancers. In a state the size of California, we would expect to see many areas that have a substantially greater (or lower) rate than the state average for at least one of these cancers, just by chance alone.

Misconception #2: Most cancers and cancer "clusters" in residential neighborhoods are caused by environmental contamination.
Fact:
The majority of known cancer risk factors are related to individual characteristics (such as age, race/ethnicity or genetic susceptibility) and behaviors (such as smoking, diet, physical inactivity, unsafe sex, and sun exposure). The relationship between cancers and environmental contamination in neighborhoods is much less established than most people realize. Nearly every investigation of a residential cancer "cluster" has failed to find a definite environmental cause.

Misconception #3: A confirmed statistical excess of cancer cases in a neighborhood proves there is contamination.
Fact:
Statistics can only tell us whether or not the cancer rate in your neighborhood is truly unusual compared to other times and places. They cannot tell us the reason why rates are unusual. Other information about your neighborhood and its residents would be needed to make sense of the situation. Even unusual rates often turn out to be due to coincidence.

Misconception #4: Neighborhood cancer concerns are investigated by testing the air, water and soil for any synthetic chemicals.
Fact:
Cancer concerns are investigated by determining whether or not there is actually an excess of cancer cases. In most cases, it turns out that there is not. If a statistically significant (see below) excess is found, the state and/or local health departments review the situation to determine if any further action is needed.

Misconception #5: Causes of cancers are well understood by scientists and doctors.
Fact: Although scientists have learned a great deal about the complex nature of cancers, much remains unknown. In populations, we know every year about how many cases of cancers we can expect, and can predict that cancer rates will be higher or lower in different groups of people. In individuals, however, no one can say for certain what exactly caused a cancer to develop.

Why are cancer rates higher in some areas than others?

There are three general reasons why cancer rates may be higher in one area than another:

  • Common risk factors: If a place has many older residents, for example, cancer rates will be higher than in a place with many younger residents. Some kinds of cancers are more common among people of one race or ethnicity than another. For example, breast cancer rates are generally higher among white women than among women of other races. Therefore, we would expect breast cancer rates to be higher in areas where the population is mostly white.
  • Coincidence: Cancer rates may be higher or lower in an area compared to the rest of the state just by chance.
  • The environment: Cancer rates could be higher in one area than another because of something in the physical environment. However, most scientists believe that environmental factors play a much smaller role than lifestyle-related factors (such as smoking and diet) or personal risk factors (such as age, family history, or race) in the development of most cancers.
How do individual characteristics and behaviors affect cancer rates in my neighborhood?

By choice or by circumstance, people tend to live in neighborhoods where they have things in common with other residents, such as race or ethnicity, education, income, occupation or lifestyle. These types of characteristics are important elements of cancer risk. Some examples include:

  • Cancer risk increases with age. Therefore, we would expect to find higher cancer rates in areas with lots of older residents.
  • Prostate cancer rates are higher among African-American men than among men of other races or ethnicities. Therefore, we would expect to find higher prostate cancer rates in a largely African-American neighborhood than in a largely Hispanic or Asian neighborhood.
  • Breast cancer rates are higher among white women than among women of other races. They also tend to be higher among women with higher socioeconomic status. Therefore, we would expect to find higher breast cancer rates in communities with many affluent white women than in other communities with a different racial and socioeconomic mix. (One reason for this difference is that women of higher socioeconomic levels more often delay child bearing and have fewer children, which in turn increases breast cancer risk.)
  • Residents of some communities may have similar lifestyle habits that increase risk, such as smoking, lack of physical activity or unhealthy diet. Therefore, we would expect to find higher rates of smoking-related cancers (such as lung cancers) in neighborhoods where a high proportion of residents are smokers than in neighborhoods with few smokers.

When researchers try to explain differences in cancer rates between neighborhoods, they take age, sex, and race into account.

How can so many cancer cases in my neighborhood be due to chance?

Chance is an explanation for cancer "clusters" more often than most people realize. Most people accept the laws of chance when it comes to games like the state lottery. It's harder for most people to accept that these same laws of chance can apply to a serious disease like cancer.

A single neighborhood has a low probability of having high cancer rates just due to bad luck (in other words, chance). However, if you look at the entire state, you would see that lots and lots of people get cancers and there are lots and lots of neighborhoods. Therefore, there is a very good probability that some neighborhoods will have this kind of bad luck. If it happens to be your neighborhood, you naturally want to know why this has happened. Most people understandably find that "bad luck" is not a very satisfying explanation and want something --or someone --to be held responsible.

Nonetheless, most experts on cancers and cancer "clusters" believe that chance is the explanation for neighborhood cancer "clusters" far more often than most people would think. "Chance" in this situation means that there is no common reason why cancers have occurred in a particular neighborhood. Given the same set of genetic and lifestyle factors, the same people probably would have gotten cancer no matter where they lived. But sometimes several people who develop cancer just happen to live in the same area -- by chance. An investigation into the cause of a chance "cluster" will never result in an answer because none exists.

Does the environment cause cancer?

Yes it can, but it plays less of a role than many people believe.

One reason for confusion about this is that the term "environment" means something different to researchers than it does to most members of the public. For researchers, the "environment" usually means any factor that is not hereditary. This may include things like cigarette smoking, diet, sunlight and infections in addition to factors in the air, soil and water. If we use this broad definition, we would say that the "environment" is a big factor in cancer risk.

However, when most members of the public think of cancer and the "environment" they are thinking of contaminants in the air, water, or soil that might cause cancers (called carcinogens). We all live in environments that contain carcinogens, and we are all exposed to a greater or lesser degree every day. The relationship between these carcinogens and cancer development is subtler, more complex and less established than most people think.

Much of what we know about these carcinogens has been learned from studies of exposures in the workplace. In the workplace, unlike in neighborhoods, chemical exposures are more often known, and occur at high levels for prolonged periods of time.

Learning more about environmental carcinogens and their role in cancer development is very important. Because their effects are complex and difficult to detect, studies of environmental carcinogens and cancer must be carefully designed and include large numbers of people. A neighborhood with a cancer "cluster" is not a very good place to study the effects of environmental carcinogens because usually the number of cases is too small, and the number of possible carcinogens is too large to be able to make a connection.

Why haven't investigations of cancer clusters been useful in identifying environmental causes of cancers?

Over the years, nearly every neighborhood cancer "cluster" investigation has failed to identify any environmental cause. This is not for lack of trying, but because cancers are very complex diseases. Methods that work well to identify the cause of an infectious disease outbreak do not work well with neighborhood cancer clusters. The reasons include:

  • Cancers take a long time to develop. For most cancers, the period between exposure to a carcinogen and the development of cancer may be ten to twenty years or more. By the time cancer rates have increased in an area, the carcinogen may no longer be measurable in the environment.
  • People change residences. In many cases, by the time a person is diagnosed with a cancer, he or she no longer lives in the area where an exposure may have occurred. Cancer registries have no way to track where people lived before or after developing a cancer.
  • Residential cancer clusters contain few cases. Even if the cancer rate in an area is found to be "statistically significant" there usually are too few cases and too many possible exposures to allow scientists to work backward to find the cause. Carcinogens that may be found in a neighborhood with several cancer cases are also likely to be found in areas without many cases.
  • It is not currently possible to determine the cause of cancer in an individual. When a person gets a cancer, doctors may suspect that certain risk factors are largely to blame. However, doctors do not have any reliable way to determine the exact cause or combination of causes for the cancer. Therefore, in a neighborhood "cluster" situation, doctors cannot be certain about what caused a cancer in any individual resident.

 

If there are a lot of pets in my neighborhood with cancer does it mean there is a problem with the environment?

The state does not track cancers in animals. Therefore, there really is no way to tell if the number of animal cancers in any particular neighborhood is truly unusual. However, we know that cancers are common especially among older animals. This is because cancer risk increases with age in animals, just as it does in people.

Humans and animals have different cancer risks. Cancers that are uncommon in humans may be very common among dogs, for example, and vice-versa. Cancer risks also vary between different animals, and even within breeds of the same species. There is no reason to suspect that groupings of different types of animals with different types of cancers have an underlying common cause.

Who should I contact with my questions about cancers in my community?

You may call either your county or local health department or the regional cancer registry that covers the county in which the community is located.

In most counties, the county health department should be contacted first. These departments will usually be familiar with local issues, and you may find that officials are already aware of the area you are concerned about. County health departments may respond to community cancer concerns themselves or refer callers to the regional registry depending upon the resources available in each county.

How will my concern be addressed?

The first step is to determine whether or not you are asking about a pattern of cancers that is really unusual. People often report cancer concerns that include lots of different types of cancers, or sometimes diseases that aren't actually cancers at all. In most cases, the epidemiologist is able to address the questions of the caller by providing current information about cancers, and to explain why the concern may not represent a real cancer excess.

Sometimes, further evaluation is necessary. The epidemiologist first works to identify the number of all observed cases of the cancer in the area and time period of concern. Then this observed number is compared to the number that would be expected based upon rates in other areas or other time periods, after adjusting for differences in the age, sex and race/ethnicity make-up of the populations. A statistical test is used to judge whether or not the difference between the number of cases observed and the number expected is likely to be due to normal random variation. If an excess number of cancer cases is confirmed, the situation is reviewed further by the health department to determine if any further investigation is needed.

Although this brief description may make this procedure sound simple, in fact it may require a considerable amount of county and registry staff time and resources, and may take several weeks to complete.