Who can get cancer?

Anyone can get cancer. One of the biggest factors that can make a person more likely to get cancer is age: 3 out of 4 cancers are found in people aged 55 or older. But there are many other factors that affect cancer risk and some of them can be changed. It is only natural that people are looking for more ways to prevent cancer.

Can cancer be prevented?

Sometimes cancer can be prevented. Looking at the whole country, it is quite possible that more than half of cancer deaths could be prevented -- if no one used tobacco and if everyone took steps to improve their health. Of course, that is a big "if."

But is there a way to guarantee that you or your loved ones won't get cancer? So far, nothing has been found that is proven to prevent every case of cancer. As of 2008, there are ways to prevent many cases of cancer in large groups of people. And there are things you can do as an individual that might reduce your chances of getting cancer. If cancer does develop, doctors also use early detection tests to improve the odds that it will be found at an early stage when it is easier to treat. But, as of today, even the best methods of reducing your chances of getting cancer (called cancer risk reduction) cannot prevent all cancers.

When you hear about something new to prevent cancer

In your quest to be healthy, you may hear about something that you are told can reduce your risk of cancer -- a new way you haven't heard about before. It sounds like a good idea, and you want to try it.

You may have questions, though, since you are thinking about spending your money, time, and energy on something that may not be proven. At this point, you may not be sure whether it will actually reduce your risk of cancer, or if it could even cause you harm. Before you put your body and money on the line, there are ways to find out more.

Methods being studied for FDA approval: Maybe the method you heard about hasn't been approved, but is "in the pipeline" to become a mainstream cancer prevention method in the future. It may take the form of a pill, a treatment, or something else. It is usually not too hard to find information about these kinds of treatments. If the treatment has ever been approved by the FDA for any medical use, you can usually find good information on risks and side effects. But it may be harder to find out about how well it works for cancer prevention.

Non-prescription herbs, supplements, diets, and special treatments: Other methods you uncover may be herbs, vitamins, other dietary supplements, health tonics, "body cleansings," or special diets that are supposed to boost the immune system, among many others. It used to be that there were almost no studies that looked at these methods, but doctors are now trying to study more of them in the same ways that they study other methods.

Lifestyle changes: There are other methods that you may find that are expected to help reduce your cancer risk. For instance, quitting tobacco, increasing the amount of fruits and vegetables you eat, getting more exercise, and losing weight are all methods that have gotten more attention lately. Studies on some these kinds of methods are fairly easy to find as well.

Whatever method you are thinking about, take the time to see what you can learn about it from sources you trust. Here we will give you some ideas to help you when you are searching for more information.

This document only addresses ways to look at information on methods that are said to be cancer preventives, although some of the same principles can be used when looking at information on cancer treatment, symptom management, and other aspects of cancer detection and care.

Consider the source

For starters, you will want to look at where your information on the cancer prevention method came from.

• Was there a report in the newspaper or news magazine?
• Was it discussed on a television or radio program?
• Did the news come from an Internet site, possibly even one that also happens to be selling the treatment?
• Was it suggested by someone in a health food store or nutrition center?
• Was there a study report in a respected, peer-reviewed medical journal such as the Lancet or the Journal of the National Cancer Institute?
• Did a friend tell you how well this new method is working for others?

News reports

Newspapers or news magazines

If you see a report in a respected newspaper or magazine, don't just look at the headlines--sometimes they can be overstated, confusing, or even misleading. You will need to read the article carefully to find out where the reporters got their information. Look for these things:

• Was this a press release from a company announcing a new breakthrough in cancer prevention?
• Was it a report from a clinical study that was given at a scientific conference?
• Was it a report from a study that was published in a respected medical journal?
• Where was the study done? What do you know about the research centers that conducted and sponsored the study?

Broadcasts

If the report was on television or radio, you will want to know if what you saw or heard can be trusted. Was the news reviewed and reported by a doctor, or was it a non-medical person such as a reporter or news anchor? Some news organizations hire medical reporters so that medical and health news can be reported more clearly to the public. Journalists without medical training don't usually understand all of the medical background and related research on the subject, so they may not be able to give a full, unbiased view.

If you heard about a study on a reliable news report, try to remember the details. Look for the kind of information that you would try to get from a newspaper (listed above), including the source of the new information. Finding these facts from broadcasts can be much harder than using printed reports, because it's hard to remember everything from a short TV or radio spot. It's also tough to go back and search for these facts after the broadcast is over. Even if you can recall everything you heard, important details may have been left out of the reports because programs have so little time to cover the subject. It may help that some news outlets post extra information or replay their newscasts on their Web sites. If you are unable to find more on the broadcaster's Web site, you may want to try contacting the TV or radio station to get your questions answered. It is better to do this right away rather than to wait. Sometimes, a question that might be answered easily a day or two after the broadcast might become impossible to find after a month or two.

Did you get the new information from a commercial or an infomercial? Keep in mind that these ads may look like news interviews, but they tell you only what they want you to hear. You have to listen carefully to learn if studies have been done, and find other reliable sources to learn more (See the "Seller promotions," section below). Some of these will cite studies without saying where they came from, or they will quote statistics from unreliable sources. Even worse, some will give information and say it came from a good source -- but when you go to look for it, the information isn't there at all.

In the section, "Consider the science," we will go into more depth about the important details you will want to get from a news report, no matter where it came from.

Anecdotal information

If someone told you that he or she (or others such as friends and family) is healthy and feeling great using this method, that is called anecdotal information. But because cancer takes so long to grow and get started, and it happens more to older people, it's fairly rare for younger people to develop cancer. Many people go through life with no sign of cancer, and those who have cancer tend to get it when they are much older. So of course you wouldn't be surprised if a large number of people hadn't developed cancer while they were using a certain prevention method, especially if they are younger or middle aged. This is why researchers follow large groups of people over many years to learn how often cancer strikes, and whether a certain type of treatment or method may lower the risk.

If you've been told someone's personal story, can you check and find out more? Keep in mind that a person may credit an herb or supplement with feeling better, even though there may be other factors involved. And sometimes, a person's belief in a method may be enough to make a person feel better for at least a short time. (See our document, Placebo Effect for more complete information on this subject.)

There are many other ways that people with good intentions can reach the wrong conclusion from a single person's experience, or even the experiences of a group of people. This is why scientists look at cancer prevention methods under such careful conditions.

Seller promotions

If the report came from an Internet seller, you may have lots of searching to do. Many of the cancer prevention methods sold there talk about the powers of herbs and supplements that have never been proven to reduce cancer risk or make people healthier. Some sellers have been caught using outright lies and fraud to make their Web site look official. A few have even made up studies or implied their product was endorsed by the American Cancer Society or approved by the Food and Drug Administration. Some have posted fake quotes from doctors, sometimes with pictures of actors dressed like doctors. Others reported studies (that were either never done or were misrepresented) they said were from well-known cancer centers.

Even though many sellers are honest, there are always a few who will go to extreme lengths. Sometimes the staff at nutrition centers and herbal shops will suggest ideas or even prescribe "immune boosting" or "cleansing" herbs to help prevent cancer and other conditions. Studies on the accuracy of recommendations from nutrition centers and herbal shops on cancer prevention methods are not yet available. But studies have been done using people who reported that they already had cancer, and who asked nutrition center staffs about possible treatments. The research showed that treatments were often suggested that were not proven to help people with cancer. In fact, some of the suggested treatments could have caused harm. (For more information on these treatments, see our document, Dietary Supplements: How to Know What is Safe.)

There are also commercials and "infomercials" that present new cancer preventives on television. These are often set up to look like news interviews, and can be very misleading since they are carefully scripted by the sellers of the product. In fact, you may learn that some of the people who sell prevention methods and other "secret cancer information" in these ways have been jailed for fraud. They may use some of the same tactics as those who market from the Internet to convince the buyer that their product will help. When such reports are first aired, they can sound so promising that people want to think there is a miracle that can protect them from cancer. It can be hard to know what to believe without more information.

What about press releases?

Sometimes a company will put out a press release about some promising substance or compound that claims to prevent cancer. This is sometimes done based on a lab study, an animal study, or a small clinical trial (a type of study in humans). Even if the press release came out after a large study was done, the company is only telling the press what they want people to hear. This is not the same as having fellow scientists take a careful look at the methods and outcome. You will want to know more about the actual study outcome, not just the parts the company wants the public to know.

What about conference presentations?

Sometimes researchers will share information with other doctors and health professionals at scientific conferences. This information is often an early look at a study that can sometimes sound very dramatic and make headlines. Again, it is important to know who is doing the study and where they work. Sometimes, the study is being done using all the careful methods of the best-run clinical trial, and the researcher is sharing his or her data with the audience. But the final outcome of these studies may not be complete at the time the findings are presented. Also, the science review that is needed before a study is published has not generally been done. You will still want to look at the final report to find out what was done and how it turned out after it was fully analyzed.

Consider the science

If you want to use proven methods to prevent cancer, you may want to know how the method was tested. The way tests are set up can affect the outcome, and sometimes can make it seem that a method or substance prevents cancer when it really doesn't.

Lab and animal studies: Pre-clinical tests

Lab studies

Scientists usually start by testing a new prevention method or treatment on cells in a dish at the lab, to find out if it has any effect there. They may treat cells with a known cancer-causing agent, for instance, and then add the new compound to see if it stops pre-cancerous changes in the cells. If it doesn't, they may change the formula or use different types of cells to try it again. Sometimes studies like this that show some effect on the cells are published as proof a cancer prevention method works. But this does not mean that it will work in humans.

This means that if you are looking at a report of a research study -- even one that says a treatment "stops cancer cells" -- you may notice that there is no mention of people. Some of these lab studies use human cancer cells, although at other times the cancer cells are from animals. (Either way, studies done on cells alone are called in vitro studies.)

At this point, anything that stops cancer cells may sound like good news. But there are many compounds that can keep cancer cells from growing in a lab dish that do not work when people take them. Some reasons a treatment may not work for people is that the substance also hurts normal cells, or maybe the body cannot absorb it and get it to the place where it is needed to stop cancer. Sometimes, even if the substance can be absorbed, can reach all the body tissues, and doesn't harm normal cells, the amount of the substance that gets to the tissues isn't enough to stop the cancer cells. There are many hurdles between lab studies and human ones.

Animal studies

If the researchers find the effect they want in cells in a dish, they may move on to animal tests. This can help them find out if the substance can be absorbed from the stomach or intestine, and how it is distributed in the animal's body. They may look for good and bad effects. Because some of these study reports are published, you may also hear about them on the news. These are called in vivo studies. This means that they were studied in living creatures.

If the study was done in animals, good outcomes again may sound promising. But methods that work in animals do not always work when they are tested on people. Animal studies often help scientists know which drugs may be toxic (dangerous or poisonous) to people, and which may show unexpected effects. Sometimes, a drug or food supplement turns out to do almost the exact same things in people as animals. But as any veterinarian can tell you, there are many drugs people use that don't work on animals, and vice versa. Some foods and drugs that are safe for animals can hurt people, and some foods and drugs that are safe for humans can hurt animals. So while animal tests can give researchers certain types of valuable information, they may not reflect how the compound will affect people.

News stories on lab and animal studies can mislead

In both lab studies and animal studies, the research report may be published. If a news group picks up the story and publishes it, they may not mention how the study was done. Often the headlines, and sometimes even the full story, do not make it clear what kind of study is being reported. Or if they do, it is mentioned in passing and or in a way that it is not noticed by the person who is listening to or reading about it. Usually, the researcher's own report will make it clear that more studies need to be done to see if the substance makes a difference in people. But the news reports on this very early research may make it sound like the compound will work in people, which can lead to confusion. This is why it helps to look at the whole printed story, and then see if you can find out more about the details of the research. Always keep in mind that there is a great deal of difference between positive results in lab or animal studies and good results in human studies.

Types of human studies on cancer risk

There are many kinds of studies that can be done in humans. Cancer prevention studies may observe people doing what they always do, and compare certain activities to their cancer risk. Or the study may offer something new (drug, method, activity, etc.) for one group of people, and then compare them to a similar group that did nothing different.

Most human studies can be classed as either studies in which people are observed (watched), or studies in which something is done (also called intervention studies).

• An observation study about cancer risk asks questions about habits and health and looks to see if there are differences in cancer risk that may be linked to these factors.

• A clinical trial on cancer risk is one in which the researchers ask healthy volunteers to do something to see if it makes a difference in their cancer risk. (This is different from a clinical study, a more general term that includes clinical trials as well as other less rigorous forms of research. Clinical studies can be medical reports describing a group of patients, or even one person's medical experience.)

Studies that observe humans

A study that simply looks at people is not really testing a method or compound to prevent cancer. The researchers are only looking at whether they can find differences in cancer risk that might be linked to something that the people already do. They can observe that some of the people who did one thing (exercise and eat fruits and vegetables, for instance) were less likely to get cancer than those who didn't. Observation studies can also find out how strongly a factor is linked to a certain disease. But a study like this can't prove that a certain thing a person did caused them to get cancer.

What is a population study?

Population studies look at large groups of people. Researchers may study how often a group gets certain types of cancer and compare their cancer risk to certain lifestyle factors. There is more than one way to do this: cohort studies follow the same group over time, and cross-sectional studies look at a group at a single point in time. Since cancer takes so long to grow, looking at the same group over time is often the best way to learn about cancer prevention methods.

Cohort studies: These studies take a group of people and watch them over time, testing them or asking them questions. (A cohort study may also be called a follow-up or longitudinal study.) A cohort study usually observes groups and compares people with different possible risk factors to see how these factors affect their outcomes.

Cohort studies can be prospective, meaning that the researchers select a group and follow them through time. They can also be retrospective, which means that the researchers find people (or medical records) and look back at the group over time. For instance, a retrospective cohort study may look back at people who were exposed to radiation in Hiroshima to find out how many of them got cancer compared to similar people who were not exposed to radiation. A prospective cohort study looking at cancer risk may keep up with a healthy group of people over time and ask them about how much they exercise, what they eat, whether they take vitamins, or even take their blood to look at chemistries. They keep this information (and often collect more) while they wait and watch to see if there is a difference in their cancer rates that can be linked back to any of these factors.

Cross sectional studies: These studies look at people at just one point in time. These studies look at how certain factors might relate to each other, but they have some drawbacks. There is usually no way to find out what happened in the past without counting on a person's memory. There is also no way to find out what happens to the people after the study.

Cross sectional studies may observe people and look for links between their actions and cancer. They often give the researchers ideas about what might be causing more cancers in some people, but these kinds of studies cannot actually show exactly what caused the cancers. Figuring out the cause requires further research, unless the link has already been proven. For example, a cross sectional study that looks at the level of a certain vitamin and cancer may find that people with lower vitamin levels are more likely to have cancer than those people with higher vitamin levels. Can we assume just from this study that the vitamin protects against cancer? No, because we can't tell from this study which came first, the cancer or the low vitamin levels. But it does give researchers ideas about what to look at next, so this kind of information can be a good place to start.

Case control studies: These studies look at people who already have a disease or condition, such as cancer, and compare them to an otherwise similar group of people who do not have the disease. Then, the researchers look at eating habits, exercise, drugs, or other factors to see if the groups are different in any ways that might explain why one group got cancer and the other didn't. Most case-control studies are retrospective (they look back at the group over time).

A common problem with these types of studies is that people often remember events or habits from years ago in different ways based on what has happened more recently. If a person has cancer, for instance, he or she may recall having had worse eating habits than those who are well. A person who is still healthy may report better eating habits in the past than what actually happened, because there is no reason to worry about them or try to remember the details. This is known as recall bias, which in this case is a type of misclassification error -- it ends up putting people into the wrong groups.

In some studies, the poor recall of those being studied may be more random in its error. For instance, suppose you ask people whether they were exposed to high doses of a certain mineral, and they have trouble remembering. In this case, there may be nothing that pushes a number of people more toward one error than the other. That could mean that a number of people end up classified as having the exposure when they didn't, and others are classified as not having the exposure when they actually did. This can dilute the groups enough that there is no difference found between the groups, even though a difference would have been found if everyone was in the right group. This is another type of misclassification error. In this case, it's another way recall bias can surface and cause false results.

When they are done well, case control studies can be helpful in producing ideas about cancer causes and risk reducers. But conclusions about cancer prevention methods based on case control studies are not as strong as those based on clinical trials.

How are observation studies misunderstood?

A study that only observes people cannot prove what factor caused an illness, but that does not stop people from trying to guess at the cause and even writing about it as if the guess was fact. For instance, there were studies some years ago that linked gum disease with heart attacks. News reports talked about this link, and there were lots of theories about how gum disease might cause heart attacks. The problem was that these were observational studies that could only show links, not find causes. The missing piece in most of the study reports was that smokers are much more prone to gum disease. A lot of people with severe gum disease smoke; smoking causes both heart disease and gum disease. So the real culprit in most cases was smoking not gum disease. Another explanation for some people may be that people who don't care for their teeth are less likely to eat well or get good preventive health care.

There are other kinds of statistical observation studies that may get a lot of attention and lead to confusion. For example, people from a country where fish is eaten 3 to 5 times a week may have a lower risk of certain diseases. A person reading about such a study might believe that the oils found in fish are responsible, and take fish oil supplements. Or they may sell the supplements, and cite the statistics as evidence. But it may turn out that the reason the people in the study had lower disease risk is that they ate less red meat, that they walked more each day, that they weighed less, or some other factor that wasn't even discussed in the study. A closer look may even reveal that the people with the smallest cancer risk were not the ones who ate more fish.

As you can see, there are often many possible explanations for these types of findings in observation studies that the reader may not know about. It's no wonder that people can be confused by the news reports on health studies.

Human testing: Clinical trials

Clinical trials must be set up carefully and all the plans reviewed ahead of time, because they propose to change something that might affect a person's life. Cancer prevention clinical trials must always be approved by a group whose job it is to look after the safety of the volunteers in the clinical trial. This group is called an Institutional Review Board or IRB. It is an independent group of doctors, statisticians, and others who review human studies to be sure that the safety and well-being of the study volunteers are protected.

Researchers may give a test substance to one group of volunteers, and either an inactive compound (placebo) or a known cancer reducer to the other group. Or researchers may test a method in one group, like aerobic exercise 4 times a week, against a group that's offered no exercise sessions. Then the groups are compared later to see if one group had more cancer than the other. There are several kinds of clinical trials that give us different kinds of information.

Safety first

Before tests of a substance that may prevent cancer can be done on humans, it is important to know if the compound being tested is safe. The results from any animal tests on the substance are reviewed along with other facts that are known about the substance.

If the substance is a known ingredient that is commonly found in food, it may be "generally recognized as safe," and be accepted for further testing. But if the amounts to be used are much higher than a person would usually get in food, further testing for safety may be needed. Other evidence may be weighed, too, such as the effects of related compounds, what is already known about the class of chemicals, and other such information. If the substance is thought to be safe, the researchers must convince an Institutional Review Board (IRB) that their methods have a chance of working, that the substance or method is safe, and that study volunteers are protected from harm.

If the compound is a new substance, the company must tell the FDA why they think it will work in humans, and share research from the lab and animal studies.

If the FDA and/or the IRB approve human testing, researchers must decide how to design clinical trials and find volunteers who are willing to take the compound or the control substance.

What is a cancer prevention clinical trial?

Clinical trials are most often thought of as research studies in which volunteers with certain illnesses or conditions help doctors find ways to treat disease or improve care. A cancer prevention clinical trial (or cancer prevention study) is a different kind of clinical trial. In these, healthy volunteers help doctors to find ways to lower the risk of certain types of cancer. There are different types of cancer prevention studies.

All cancer prevention clinical trials are done to answer these questions:

• Does the medicine or supplement or other cancer prevention method work to prevent cancer?
• How safe is it to take the study agent or use the cancer prevention method?
• Does it reduce the death rate in the group that uses the cancer prevention method by reducing the number of people who get the cancer?

Some prevention studies may require the volunteer to do something, like exercise for a certain length of time each day, stop smoking, eat extra fruits and vegetables, or get tests to find and remove pre-cancers. The action may either be to avoid something thought to be harmful, or to do something thought to be helpful. These are called action studies, and they help researchers find out if the actions that are taken will reduce cancer risk.

In another type of study, the volunteer must take something, such as a drug, vitamin, mineral, or food supplement, to see if it reduces their cancer risk. Scientists who conduct these studies want to learn whether the medicine or supplement (often called a study agent) reduces cancer risk. They will also look at the safety of the study agent. These are called agent studies or chemoprevention studies.

Cancer prevention clinical trials that involve a drug, supplement, vitamin, or mineral are set up something like the clinical trials that test new drugs. They are generally done in 3 phases, starting with phase I. Each phase is designed to answer different questions about the compound being tested:

• Phase I trials look for the best way to give the compound, the best dose, and check for any harmful side effects.

• Phase II trials look at whether the substance has an effect in preventing cancer.

• Phase III trials compare a promising new compound to one that is already in use, by giving the new one to one group and the older one to the other group (see "Control group" below.) If there is no substance that's already being used for the same purpose, the control group may get a placebo (a sham pill or supplement; see "Placebo" below).

These phases build on each other. If, for instance, the side effects are severe in the phase I trial, a phase II trial is not likely to be done. If phase I shows no harmful effects, a phase II study may be carried out. But if the phase II study shows no effect, a phase III trial is not planned, unless there is reason to doubt the phase II outcome. The phase I part of the clinical trial is usually the smallest of the study groups, and phase III the largest. If you can find only phase I studies on a cancer prevention method, it may mean that there is not yet good information about its use in humans.

A cancer prevention study may be done on different kinds of volunteers, depending on its purpose:

• Clinical trials that look for ways to reduce cancer risk in people who have never had cancer
• Clinical trials that look for ways reduce cancer risk in people who have already had cancer

Sometimes, a cancer prevention study may enroll volunteers who are known to be at higher risk of cancer. Those at higher risk stand to benefit more from anything that helps to reduce risk. Also, if the method actually helps, it often takes less time to see if risk is lowered in those who are likely to have more cancers.

Controlled clinical trials of cancer prevention

A clinical trial is best done with at least 2 groups that are very much alike. This helps researchers to know that any differences between the groups are actually due to the method being tested, rather than factors that the volunteers may have had when they came into the study. These 2 groups are called the intervention group and the control group. Sometimes, there will be more than 2 groups, when 2 or more interventions are being tested against the control group.

Intervention group. This is the group taking the study agent, or the group that does the action. It is also called the test group or study group.

Control group. In the study of a compound to prevent cancer, the control group takes either:

• a standard agent that's being compared with the study agent (the substance being tested)
• a look-alike pill or substance that contains no active ingredient, called a placebo.

In an action study, the control group may:

• get standard health promotion, such as information or instructions about eating or exercise
• do something other than the action taken by the test group
• do nothing or be put on a delayed treatment list ("wait list")

No matter what the control group members do, they are watched in the same way the test group is during and after the intervention.

Protocols. Clinical trials follow strict guidelines that help make sure the science is sound and that volunteers who take part are protected. Each clinical trial has a principal investigator who is in charge of the study. Each study also has a plan, called a protocol, which says what the study will do and how it will do it. The protocol explains the study design, who can and can't be in the study, what is being studied, what medical tests are needed and how often, and what other information will be gathered.

A closer look at the evidence

If you are able to find clinical trials that were done on the method you are looking at, it is important to notice what kind of study was done, and see what was compared. You will also want to look at some other factors in the study.

Study subjects: If you happened to get your information from a study done on people, this is a good start. But there are many stages a treatment must go through in human tests before it can be used by most doctors to prevent cancer. It is possible that the study is an early (preliminary) one, or a pilot study. These are small early studies, in which a drug or treatment is tested on a few people just to get an idea if it is worth testing on larger numbers of people. (See the "Clinical trials" section above.)

Control group: A study that has a control group is called "controlled." This may mean that the cancer prevention study was carefully planned, and that people who got the prevention method were compared to others like them who did not get that prevention method.

Studies that do not have control groups may compare their disease rates with older studies or general information collected on other groups. But these may not offer good comparisons due to differences in the groups of people which can affect how much cancer will be found. For instance, one group may span different ages, which affects how many people will get cancer during the study period. Different parts of the country have more cases of certain cancers. Some cancers affect one sex more than the other. Some regions (and even entire states) have a higher percentage of smokers than others. And certain subgroups get more exercise and eat healthier than others. These, and many more factors, make it a bad idea to compare a test group to others chosen in a different way or from a different pool of people than the test group.

The best control group is like the test group in every way other than the factor being studied. That is why better-planned studies try start with one group of people and randomly divide them into 2 or more groups, as described below.

Randomization: This means that the prevention method is compared using similar groups of volunteers who were chosen completely by chance to be in one group or the other ("randomized"). This reduces the risk, for instance, that the older people who are at higher risk for cancer mostly end up in one group, which could change the study outcome.

Some other examples of the benefits of randomization: If more young people who start out healthier end up in the group getting the new prevention method, it may make the prevention method look better than it really is. If more people who started out with a higher risk of cancer (such as smokers) end up in the new cancer prevention group, that group may fare worse than the control group. This could make the prevention method look less effective, because it was tested on people who were more likely to get cancer. Of course, if more smokers end up in the control group, they may make the test method look better because the control group will likely get more cancer over the years.

To avoid problems like these, researchers put people into one group or the other by choosing people for each group using methods along the lines of flipping a coin -- usually with a computer program. Randomization lowers the odds that one group will be very different from the other. This is why you don't know, when you agree to take part in a randomized controlled cancer prevention trial, whether you will get a standard prevention or the new one that is being tested. And since there are not many known standard preventions for cancer, you may very well end up in a placebo group. When you are informed about the cancer prevention clinical trial, the study team should let you know if there is a chance you will be in a placebo group.

Keep in mind that this is very different from clinical trials in which people already have cancer. When treatment clinical trials are randomized, current treatments (rather than placebos) are used in the control group. This lets the researchers know whether the new treatment works better than the one that is now being used.

Blinding: This means that the patient does not know which cancer prevention he or she is getting. If the patient knows the cancer prevention that he or she is getting, the study is called an "open label" study. One advantage to a blinded study is that it can help the researchers learn more about side effects. For instance, if patients know that they are getting placebos, or that they are getting a vitamin or a known standard treatment, they might not bother to report health problems to the study coordinator. Those who know they may be getting the drug are more likely to report nausea, headaches, and fever, even if the problems turn out to be from food poisoning or the flu. It isn't just everyday problems -- the same can be said for serious illnesses, which sometimes happen with no known reason.

You can see that if the treatment group mostly reports everyday health problems and the control group generally doesn’t, it can make the treatment method look like it has a lot more side effects. This is just one of the ways a patient's knowledge about what he or she is taking can affect a study's outcome.

Double blinding: This means that neither the researchers nor the patient knows which treatment the patient is getting until after the prevention trial is completed and the observations are on record. This helps to avoid bias in which a researcher expects one group of patients to do better, which can affect the researcher's observations. In cancer prevention trials, observations are carefully measured and written up. After that, they break the code to find out who was in which group, so that the people analyzing the data can find out which group (if any) did better than the other.

There is an exception to this rule, however. In studies where there is a chance that some harm might take place, a Data and Safety Monitoring group follows the results of the study. They do not share this information with others unless it appears that harm is being done. For instance, if one group appears to be doing much better or worse than the other after an early review, they may require that the study be un-blinded so that a closer look can be taken at what may be going on. If the study is found to be causing harm (either to those getting the prevention or those not getting it), the study may be stopped before its scheduled ending time.

Statistical significance: The data are carefully looked at to see if the difference between the groups is likely to be due to chance. This is called a "test of statistical significance." It means that if one group came out better than the other by a large enough margin, it is very unlikely that the differences were by chance, and the results are said to be "significant." Keep in mind this kind of test alone cannot prove that factors besides random chance didn't bias or confound the results. Careful study planning and precise measurements are used to avoid those factors.

Publication and peer review: Publishing the findings in a respected peer-reviewed journal means that the methods and information from the study were looked at by other doctors or scientists. When they look at the information, they want to be sure that the scientific procedures were properly followed. They also keep an eye out for any bias or other factors that would make one group do better than the other for some reason other than the treatment being studied.

The highest standard of proof that a cancer prevention method works is a double-blind randomized clinical trial on humans that has met the strictest standards of scientific method. If blinded studies are not possible, scientific procedures must still be carefully followed to be sure that any difference in outcomes are due to the treatment. This usually allows the study to be published in a respected, peer-reviewed medical journal.

It takes more than one study to prove something really works. Even breakthrough ideas take a lot of testing to show that they work. Since many good ideas don't pan out for cancer prevention, the failure rate can be high. One study with a good outcome doesn't mean a cancer prevention method works. Even if a study is done in the most careful manner, future studies that try the same thing sometimes find that they get different results. This can happen because the second clinical trial tests the method on a different group of people that does not respond the same way as the first group. Or the method may be used in a slightly different way, in a different dose, or with some other difference that may have not even been noticed.

Science builds on the studies in the lab, and sometimes tests in animals. If the cancer prevention method seems to be safe, it is moved up to test in a small group of people. Getting to this point often takes years. If these results look promising, a phase I clinical trial may be started. At any point, the researchers may find that the cancer prevention method really doesn't work as they thought it would. But even if it does, good testing can take some time.

What if different clinical trials show different outcomes? This can be very confusing, especially at first. When there are just a few studies, as there may be on a compound that is generally thought to be safe, tests on humans may be the first type done. There may not be much understanding of how it might work from lab studies or animal studies. Even when the studies are set up well, these clinical trials often end up showing very little difference, if any, between the people who took it and those who didn't. When the compound really doesn't have any effect, chance will often tip the scales in one direction or another -- sometimes even enough that the results look significant. This means that sometimes the placebo group will do a bit better than the test group, while at other times, the group that gets the new compound does a little better. When results conflict with one another like this, it often means that the treatment has very little effect. Or it can be study design problems, or other factors that affected the outcomes.

Publication bias: There is another problem that can creep in as studies are published. Sometimes, the studies that show no difference between the treatment and placebo, or the ones that show the placebo group doing better, are not published. After all, it isn't exactly exciting news when something doesn't work. But these kinds of studies could really help people who are trying to decide whether it is worthwhile to take the treatment. Worse, if the only clinical trials are published are the ones that show the treatment helps, a person reviewing the published information might not be able to find studies that showed no difference. He or she might conclude that the treatment was helpful, because those are the only studies that were published. This is an example of what is called publication bias.

Other questions about studies on new ways to prevent cancer

Does the study make sense in light of what is known about the human body and the way the method affects it?

This question is often called biological plausibility. For instance, a substance that blocks female hormones may be expected to reduce the risk of cancers that use female hormones to grow. (Of course, whether it would really reduce cancer risk in people would still need to be tested.) But sometimes the researchers may not understand exactly how a substance may work in the body. In cases where little is known about how it may work, evidence from earlier studies may have a role in suggesting what the substance is likely to do in humans.

Does the study support or contradict past studies?

The more evidence there is for a prevention method, the more likely the results are to be true. Sometimes, a study or two will come out that encourages people to believe that a certain food or supplement will reduce cancer risk. Later on, more careful studies have often found that the lower cancer risk was due to something else entirely -- chance, age, healthy habits -- and had nothing to do with the food or supplement.

Does the study promote something that is supposed to prevent all cancers?

This goes along with biological plausibility, discussed above. Since there are many different types of cancer, and many are known to be caused or affected by different factors, it is very unlikely that one method can address all of them. Claims that there is one method that prevents all kinds of cancer are highly suspect.

Why are most products that are advertised as immune boosters and cancer preventives not approved by the FDA?

There are many herbs and food extracts that are advertised as having an effect on cancer. As long as these food-related products are generally regarded as safe, there are relatively few restrictions on their sales. Many are simply packaged and sold. Because some of these supplements have been found not to contain what is listed on the label, and others have been found to include substances that were not on the label, the FDA set up new rules for dietary supplements in 2007. This law will not be fully in effect until 2010, but it is intended to help be sure that the supplement contains what it says on the label, with no extra ingredients or impurities. This law will still not require those who make or sell the product submit proof that the herb or supplement is safe or effective. The law does not address the supplements' effects on the body.

Since there is growing interest in supplements, researchers have started studying some of them using the same methods used for cancer treatments and mainstream cancer prevention methods. Large sums of money are not usually available to study herbs and vitamins, so at first these studies tend to be smaller. But because the safety of the substance is not usually called into question, there is not as much need for animal testing. When looking at studies of these herbs or supplements in people, you would want to look at the same questions as you would for cancer prevention clinical trials.

On the Internet, in conferences, and in health food stores, those who sell herbs will sometimes try to use lab studies or animal studies showing that the substance blocks cancer cells as evidence that the herbs work. Some sellers will refer to studies that are not published in peer-reviewed journals. The studies may be written up in a "natural cures" book or posted on an Internet Web site. These leave you with no assurance that the studies were done as they are presented.

It also happens that science researchers will isolate a chemical from an herb and test it in the lab to find out if it affects cells. We know that sometimes the effects of the isolated chemical might be different from the effects of the whole herb (especially in large doses). This is why researchers may have to prove that the extract is safe before testing it in humans. This type of study is usually published in scientific literature and can be found there.

There is a down side to tests of specific herbal extracts. If sellers of an herb know about scientific studies done with herbal extracts, some of them may talk about the study's findings as if the study's success means the whole herb works the same way. Purified extracts are just one part of the herb and do not work the same way as the whole herb, and the two types of studies cannot stand in for one another.

What does this mean to you?

After going over what all of these things mean, here are some questions about new treatments you will want answered:

• Was the new cancer prevention method tested in the lab (on cells in a dish), in animals, or in humans?

• Who did the study? Was it done by known researchers and cancer treatment centers?

• Are there other studies that were done before that support this outcome?

• If the study was done in humans, how many were involved? How long were they followed?

• Was there a control group (people who got placebo or another prevention method)?

• Were people chosen at random to be in either the test group or the control group (randomization)?

• Was the study blinded (were the patients and/or researchers kept from knowing who got which prevention method while the patient was being watched for the effects)?

• Was there a difference in outcome between the group getting the new method and the group getting the placebo/standard prevention?

• Was the difference in outcome measured in numbers of new cancer cases?

• Was the study published in a respected, peer-reviewed journal? Was it presented at a conference or sent out in a press release?

• Is the prevention method likely to be harmful to me? What is known about drawbacks or side effects?

What if I can't find much evidence but I still want to try it?

Of course, even if the evidence is not there for your cancer prevention method, you may decide to use it anyway.

If you choose to use unproven measures, talk with your doctor about it to get another opinion about the method. If it is a vitamin, herb, or supplement, see what you can find out about side effects, allergies, and other possible effects. You will also need to let your doctor know about it in case it causes problems with other medicines you are taking. Often, studies of "natural remedies" do not collect this sort of information, and it can be hard to find. We encourage you to learn all you can. You can always call your American Cancer Society. We can almost always help you get more information on any treatment or method you are considering.

Recommended ways to reduce your cancer risk

About 1 in 3 cancer deaths in the United States each year is related to diet, exercise, and overweight. Another 1 in 3 cancer deaths are due to exposure to tobacco.

How many ways can we reduce cancer risk today? Here are some methods that have a good deal of science to back them up.

American Cancer Society recommendations for individual choices about nutrition and physical activity

Maintain a healthy weight throughout life.

• Balance calorie intake with physical activity.
• Avoid excessive weight gain throughout life.
• Get to and stay at a healthy weight if currently overweight or obese.

Adopt a physically active lifestyle.

• Adults: Engage in at least 30 minutes of moderate to vigorous physical activity, beyond your usual activities, on 5 or more days of the week. And 45 to 60 minutes of intentional physical activity on 5 or more days a week is even better.

• Children and adolescents: Engage in at least 60 minutes per day of moderate to vigorous physical activity at least 5 days per week.

Moderate activities are those that require about as much effort as a brisk walk. Vigorous activities generally use large muscle groups and raise your heart rate, speed up your breathing, and make you sweat.

Eat a healthy diet, with an emphasis on plant sources.

• Choose foods and drinks in amounts that help you get to and stay at a healthy weight.
• Eat 5 or more servings of a variety of vegetables and fruits each day.
• Choose whole grains over processed (refined) grains.
• Limit intake of processed and red meats.

If you drink alcoholic beverages, limit your intake.

• Limit your intake no more than 1 drink per day for women or 2 per day for men.

American Cancer Society recommendation for community action on nutrition and physical activity

Public, private, and community organizations should work to create social and physical environments that help people adopt and maintain healthful nutrition and physical activity:

• Increase access to healthful foods in schools, worksites, and communities.
• Provide safe, enjoyable spaces for physical activity in schools.
• Provide for safe, physically active transportation (such as biking and walking) and recreation in communities.

Avoid things that cause cancer

• Avoid smoking, second hand smoke, and all other forms of tobacco
• Don't expose yourself to other known cancer causing agents (carcinogens) Learn more about chemicals or agents that you work with or use at home, and how to protect yourself. (See our document, Known and Probable Human Carcinogens.)
• Protect yourself from sunlight and other UV light sources (tanning beds and lamps)

Get tested for common cancers and pre-cancers

Use early detection methods that can find pre-cancerous changes in some parts of the body. Treating these pre-cancers actually keeps them from growing into cancer:

• Pap tests for women as recommended
• Colonoscopy, CT colonography, or sigmoidoscopy for people 50 and over (or earlier if high risk)

For more on prevention and early detection, see our documents, Cancer Prevention and Early Detection Checklist for Men or Cancer Prevention and Early Detection Checklist for Women.

Even though some of these methods don't prevent cancer, early detection methods can help lower your chance of dying from cancer, and improve your chance of a cure. See American Cancer Society Guidelines for the Early Detection of Cancer and see a doctor to discuss the best plan for early cancer detection in your case.

What about heredity? How does it affect my cancer risk?

All cancers involve damage to genes that control the cell's growth (division). But only about 1 in 20 cancers are due to damaged gene that is inherited from a parent. If you inherit a damaged gene, you can have a very high risk of getting one or more types of cancer. There are genetic tests you can take to find out if there is an inherited problem if a certain type of cancer is common in your family. (See our document, Genetic Testing: What You Need To Know for more on how this works.)

This means 19 out of 20 cancers are not due to inheriting damaged genes, but from damage that builds up over your lifetime. Mutations in your genes may be caused by internal factors such as hormones or the way nutrients are processed within cells. Or they can be caused by external factors such as tobacco, chemicals, and sunlight.

If you think you are at high risk for certain types of cancer, talk with your doctor about whether earlier screening or extra testing is needed. In some cases, there are medicines that can reduce your risk. (See our document, Medicines to Reduce Breast Cancer Risk.)

Using unproven methods that may reduce your risk

For those who are still searching for a guarantee -- or even a boost in the right direction -- there is no shortage of other ideas as to how a person might be able to keep from getting cancer. Some ways have been studied, while most others have not. Some methods have proven safe, while the safety of others is still unknown. But people want to be as healthy as possible, and if they find something that may help, they may want to try it even if there is no evidence. With thousands of possibilities that take years to study, there will always be theories waiting to be checked out.

The American Cancer Society supports the right of people to decide what is best for them. But we encourage people to discuss prevention methods or treatments they may be thinking about with their doctors and other health care providers. We also encourage people to consider using methods that have either been proven to work or are being studied in clinical trials.

Additional resources

More information from your American Cancer Society

We have selected some related information that may also be helpful to you. These materials may be ordered from our toll-free number, 1-800-ACS-2345 (1-800-277-2345).

• American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention (also available in Spanish)

• What is Cancer? (also available in Spanish)

• Complementary and Alternative Methods for Cancer Management (also available in Spanish)

• Guidelines for Using Complementary and Alternative Methods

• Dietary Supplements: How to Know What Is Safe

• Placebo Effect

• Medicines to Reduce Breast Cancer Risk (also available in Spanish)

• Known and Probable Human Carcinogens

• Skin Cancer Prevention and Early Detection (also available in Spanish)

• Summary of American Cancer Society Recommendations for Human Papillomavirus (HPV) Vaccine Use to Prevent Cervical Cancer and Pre-Cancers (also available in Spanish)

• Cervical Cancer: Prevention and Early Detection (also available in Spanish)

• American Cancer Society Guidelines for the Early Detection of Cancer (also available in Spanish)

• Genetic Testing: What You Need to Know

• Clinical Trials: What You Need to Know (also available in Spanish)

National organizations and Web sites*

Along with the American Cancer Society, other sources of patient information and support include:

First Gov for Consumers: Health
Web site: http://www.consumer.gov/health.htm
(includes information and many links about health, including how to spot health fraud)

Food and Drug Administration
Toll free number: 1-888-463-6332 (1-888-INFO FDA)
Web site: www.fda.gov

National Cancer Institute
Toll free number: 1-800-422-6237 (1-800-4-CANCER)
Web site: www.cancer.gov

*Inclusion on this list does not imply endorsement by the American Cancer Society.

No matter who you are, we can help. Contact us anytime, day or night, for information and support. Call us at 1-800-ACS-2345 or visit www.cancer.org.

References

American Cancer Society. Cancer Facts and Figures 2008. Atlanta, GA: American Cancer Society; 2008.

Barnett ML, Hyman JJ. Challenges in interpreting study results: The conflict between appearance and reality. 2006. J Am Dent Assoc, Vol 137, No suppl_2, 32S-36S.

Gotay CC, Dumitriu D. Health food store recommendations for breast cancer patients. Arch Fam Med. 2000;9:692-699.

Hoffer, LJ. Complementary or alternative medicine: the need for plausibility. Canadian Med Assoc J. 2003. 168 (2).

Kushi LH, Byers T, Doyle C, Bandera EV, McCullough M, McTiernan A, Gansler T, Andrews KS, Thun MJ; for the American Cancer Society 2006 Nutrition and Physical Activity Guidelines Advisory Committee. American Cancer Society Guidelines on Nutrition and Physical Activity for cancer prevention: reducing the risk of cancer with healthy food choices and physical activity. CA Cancer J Clin. 2006;56:254-281.

National Cancer Institute. Cancer Prevention Overview (PDQ®), 2007. Accessed at www.cancer.gov/cancertopics/pdq/prevention/overview/HealthProfessional on January 13, 2009.

National Cancer Institute. Data and Safety Monitoring Guidelines. Accessed at www.cancer.gov/clinicaltrials/conducting/dsm-guidelines/allpages on December 12, 2008.

National Cancer Institute. Taking part in clinical trials: Cancer prevention studies. Accessed at: www.cancer.gov/clinicaltrials/resources/taking-part-prevention-trials on March 11, 2008.

Simon SD. Statistical evidence in medical trials: What do the data really tell us? 2006. Oxford: Oxford University Press.

Stampfer MJ, Skerrett PJ. Health for Life: Whom Can You Believe? 2006. Newsweek, Jan 16.

US District Attorney's Office, South District of Florida, Press Release. Arthur Vanmoor charged in superseding indictment for selling fake cancer cure, 2007. Accessed at http://www.usdoj.gov/usao/fls/PressReleases/070827-01.html on January 13, 2009.

Weed DL, Gorelic LS. The practice of causal inference in cancer epidemiology. Cancer Epidemiol Biomarkers Prev. 1996;5:303-311.

Last Medical Review: 03/16/2009
Last Revised: 03/16/2009