Cases of Cancer Clusters

A well-known cancer cluster was discovered in the 1960s and continues to be studied today. The use of asbestos, a naturally occurring mineral, was prevalent in factories, shipyards and building products throughout the 1950s and 1960s. Many individuals who were exposed to asbestos on the job developed a disease known as mesothelioma. Many individuals also developed lung cancer. These diseases were directly linked to asbestos exposure and this exposure is still the largest known cause of mesothelioma in the world today.

Between 1941 and 1971 many women were exposed to a drug called diethylstilbestrol (DES). This synthetic hormone was prescribed for women to prevent miscarriage. It was also used to prevent breast engorgement during pregnancy. Over 10 million women were exposed to the hormone and the cancer cluster was discovered after many daughters developed vaginal clear cell carcinoma. This rare cancer is directly linked to exposure in the womb to DES. It is the largest known cause of vaginal clear cell carcinoma.

These two cases are examples of cancer clusters in very specific situations with clear-cut exposures and development of very rare forms of cancer. Other historical cases include the development of leukemia and lymphoma in chemical workers who were exposed to the chemical benzene, dye workers exposed to aniline compounds who developed bladder cancer and farmers who developed skin cancer after extreme exposure to the sun.

Cancer is not the only disease that can sometimes occur in clusters. Other disease clusters have also helped to identify environmental hazards like bacteria contaminated foods, and a respiratory illness now known as SARS. Epidemiologists study these types of environmental factors as well as those known to cause cancer.

Heredity and Environment in Relationship to Cancer Clusters

Heredity and environment are the main contributing factors to the development of cancer. Epidemiologists focus on these two areas when studying cancer clusters. Behavior and lifestyle also contribute to cancer risks so they are also included when studying environment.

To establish a connection between an environmental risk factor and a genetic predisposition is difficult. Scientists must study large groups of people over the course of a long period of time to understand the process of carcinogenesis (the process of normal cells becoming cancer cells). This makes establishing the existence of cancer clusters a lengthy process.

The exposure to certain carcinogenics may also not affect health for many years. In the case of asbestos exposure, it often takes decades for mesothelioma to develop after exposure to the substance. Therefore, the cancer cluster was not identified until many years after the initial exposure began.


Researchers have made some discoveries in regards to heredity and cancer. They have determined that all cancers are caused by some form of gene mutation, which causes the gene to perform abnormally.

Some of these abnormalities are present in an individual at birth. These types of gene mutations are passed down from parent to child. Although this is an uncommon source of cancer it does create “inherited susceptibility” or a predisposition to cancer. Inherited susceptibility does not mean cancer will definitely occur in an individual, however it does indicate a condition where, if other exposures and factors exist, cancer may occur.

Cancer clusters can often be suspected within families due to a higher than expected rate of disease within the family. These cases can sometimes be attributed solely to coincidence. Other times these “familial clusters” can be due to inherited susceptibility coupled with environmental factors.

Somatic alterations are changes that occur to genes throughout an individuals lifetime. These alterations are more likely than inherited susceptibility to be the cause of cancer.


Environmental exposures have often been suspected by individuals to be the cause of cancer clusters. The environmental factors that are considered by epidemiologists include the air, water and earth surrounding an individual or group. Other environmental factors that influence cancer clusters are home and workplace conditions, tobacco use, drugs and alcohol, chemical exposures, and exposure to radiation or even sunlight.

Due to varying susceptibility in individuals and the vast array of chemical and other substances in our world identifying specific causes of cancer is extremely difficult.

Workplace discoveries in regards to chemical exposures have provided positive results in the study of dangerous exposures to certain substances. Certain occupational studies have proved the toxicity of chemical carcinogens and led researchers to develop ways of reducing exposure.

Causes of Cancer Clusters

There are many risk factors for cancer in the world today. Smoking, poor diet, lack of exercise and inactivity are some of the preventable cancer risks. Carcinogens in the environment can also cause cancer. Cancer clusters, once identified as such, often may not have an underlying cause.

There are often several explanations when considering cancer clusters including coincidence, external behaviors (smoking) and miscalculation of historic incidences. When these factors do not explain the cancer cluster, a deeper, more thorough study can be conducted. These often take many years and do not always provide a clear and definitive answer.

Epidemiology and Cancer Clusters

Epidemiology is the study of the factors that cause illnesses and disease with in a population. This field of study is critical in the prevention and spread of communicable and non-communicable disease. Epidemiology is used to identify disease from the outbreak through diagnosis and treatment.

Epidemiolgists, those scientists who study the causes of disease, utilize knowledge in biology, sociology and philosophy in an effort to understand and communicate risk factors for disease.

Carcinogenesis, or the process by which normal cells become cancer cells, is studied by epidemiologists to determine the existence of cancer clusters.

Cancer clusters are rare and epidemiologists determine, through specific criteria if and when a cancer cluster exists. Many individuals will report a cancer cluster but it is the job of an epidemiologist to determine if there is an environmental cause for the incidence of cancer or if the high rate of cancer is merely coincidence.

An epidemiologist will utilize several specific criteria to decide if further investigation is necessary in regards to a cancer cluster. When one type of cancer is found in a population in large numbers this may be a warning sign of a cancer cluster. There are over 100 varieties of the disease so it is rare for one, specific type to be found in large numbers in a population.

There are rare forms of cancer and there are more common forms of the disease. If a rare form of cancer is found in high numbers within a community or group this may also signify a cancer cluster.

A third warning sign to the existence of a possible cancer cluster is when a specific age group in an area suffers from a form of cancer that does not usually affect that age group.

In the beginning stages of determining the existence of a cancer cluster, epidemiologists must first assess the health of the individuals affected. If the cancer is a result of an underlying infection or due to the spread of cancer from another area of the body, this individual is not part of a cancer cluster. The primary cancer is the only cancer considered in the study of a cancer cluster.

Epidemiologists will use biological knowledge to study the significance and causes of the particular type of cancer. The scientist will use knowledge of biological causes of disease to determine if exposure to environmental factors has the ability to cause the specific type of cancer.

In the process of deciding whether or not a cancer cluster truly exists it is important for the epidemiologist to define the borders of the geographical area. Often there are cases in outlying areas causing additional individuals to become involved in the study. These situations may create false cancer clusters.

It is very difficult to determine with absolute certainty if the number of cancer cases is higher that what would be normally expected. The use of statistical factors such as age, gender and race are utilized but lifestyle factors may cause the numbers to be different than what would normally be expected.

It is not possible in all cases to make a determination that a cancer cluster does exist. In some situations the cases of cancer are higher than what would normally be expected within the geographical area in the specified time period, however the epidemiologists cannot find an underlying cause for the disease.

In some situations the study of a cancer cluster cannot be completed due to the small amount of subject cases. The epidemiologist must have enough cases of the disease to provide accurate conclusions.

Even in situations when there are many cases, the history of the individuals involved may make specific determinations regarding exposure impossible. For example, an individual may have lived in many areas over the course of a lifetime and determining when and where exposure occurred would then be impossible.

Factors Affecting Cancer Rates

The age of a group of individuals will affect the rate of cancer development. Individuals over the age of 45 have a much greater risk of developing cancer than people younger than 45 years of age. Men and women over the age of 60 have an even greater risk of developing some form of cancer. It would stand to reason that these age groups would have more cases of cancer than other groups in younger age brackets.

Other factors that may affect the rates of cancer development in a particular group are lifestyle behaviors. Individuals with a poor diet may experience a higher cancer rate than those who eat healthfully. A diet rich in vegetables, fruit, lean meats and fish and healthy fats can help prevent cancer in some individuals.

Exercise is also an important part of keeping cancer at bay. Individuals who do not participate in any regular form of exercise are at a greater risk for developing cancer. According to the American Cancer Society one third of all cancer deaths are in some way related to nutrition, obesity, and/or physical inactivity, and could possibly be prevented.

In one study conducted by Dr. Christine Friedenreich, of Alberta Health Services-Alberta Cancer Board in Calgary, Canada, it was discovered that women with breast cancer who had exercised more than four hours per week over their lifetime had a 44 percent lower risk of dying from the disease. Other studies have found similar results with other forms of the disease.

“Greater participation in physical activity has consistently been associated with reduced risk of cancer incidence at several sites, including breast and colon cancers,” said James McClain, Ph.D., cancer prevention fellow at the National Cancer Institute.

Another enormously risky lifestyle behavior is tobacco usage. It is estimated that discontinuing the use of tobacco could prevent one third of all cancers.

These lifestyle factors may cause the incidence of cancer to increase in certain groups such as families or employees that participate in these high-risk behaviors. Cancer clusters may seem to exist however it is lifestyle behaviors, not external environmental carcinogens that cause the unusually high incidence of cancer in these groups.

Cancer Cluster Description and Factors

The term “cancer cluster” describes the condition when the incidence of cancer occurs in higher numbers than expected in a specific geographic area within a particular time frame. These clusters may also be particular to a workplace or a group of people, such as a family.

Families or members of a community often report cancer clusters when cancer is diagnosed in more than one individual in the group. The individuals often feel the need to explain the reason behind the cancer diagnosis and sometimes look at chemicals in the environment to explain the cause of the cancer diagnosis.

It is rare for these cases to be an actual cancer cluster and more often the situation is just coincidental. The forms of cancer are often not even related.

The high incidence of cancer in the United States and worldwide makes it common for individuals in a specific geographic area or even in a shared workplace to get cancer. If the cancer rate continues as it is today at least one in every 10 children born will acquire cancer within their lifetime. This fact could explain the phenomenon of cancer occurring repeatedly in geographic areas or within certain groups of people.

There are over 100  types of cancer. Each form of the disease may have a different cause as well as different rates of occurrence and survival. Some forms of cancer are caused by environmental factors but many others may be caused by genetics and some cases are caused by a combination of both factors. Each type of cancer occurs and reacts in very different ways therefore it is not reasonable to believe a common cause is to blame for every occurrence in a particular group.