Cancer Prevention Overview (PDQ®)–Patient Version

Cancer researcher SCOR NCORPCancer prevention is action taken to lower the chance of getting cancer. In 2020, more than 1.8 million people will be diagnosed with cancer in the United States. In addition to the physical problems and emotional distress caused by cancer, the high costs of care are also a burden to patients, their families, and to the public. By preventing cancer, the number of new cases of cancer is lowered. Hopefully, this will reduce the burden of cancer and lower the number of deaths caused by cancer.

Cancer is not a single disease but a group of related diseases. Many things in our genes, our lifestyle, and the environment around us may increase or decrease our risk of getting cancer.

Scientists are studying many different ways to help prevent cancer, including the following:

  • Ways to avoid or control things known to cause cancer.
  • Changes in diet and lifestyle.
  • Finding precancerous conditions early. Precancerous conditions are conditions that may become cancer.
  • Chemoprevention (medicines to treat a precancerous condition or to keep cancer from starting).
  • Risk-reducing surgery.

Carcinogenesis

KEY POINTS

  • Carcinogenesis is the process in which normal cells turn into cancer cells.
  • Changes (mutations) in genes occur during carcinogenesis.

Carcinogenesis is the process in which normal cells turn into cancer cells.

Carcinogenesis is the series of steps that take place as a normal cell becomes a cancer cell. Cells are the smallest units of the body and they make up the body’s tissues. Each cell contains genes that guide the way the body grows, develops, and repairs itself. There are many genes that control whether a cell lives or dies, divides (multiplies), or takes on special functions, such as becoming a nerve cell or a muscle cell.

Changes (mutations) in genes occur during carcinogenesis.

Changes (mutations) in genes can cause normal controls in cells to break down. When this happens, cells do not die when they should and new cells are produced when the body does not need them. The buildup of extra cells may cause a mass (tumor) to form.

Tumors can be benign or malignant (cancerous). Malignant tumor cells invade nearby tissues and spread to other parts of the body. Benign tumor cells do not invade nearby tissues or spread.

Risk Factors

KEY POINTS

  • Factors That are Known to Increase the Risk of Cancer
    • Cigarette Smoking and Tobacco Use
    • Infections
    • Radiation
    • Immunosuppressive Medicines After Organ Transplant
  • Factors That May Affect the Risk of Cancer
    • Diet
    • Alcohol
    • Physical Activity
    • Obesity
    • Diabetes
    • Environmental Risk Factors

Scientists study risk factors and protective factors to find ways to prevent new cancers from starting. Anything that increases your chance of developing cancer is called a cancer risk factor; anything that decreases your chance of developing cancer is called a cancer protective factor.

Some risk factors for cancer can be avoided, but many cannot. For example, both smoking and inheriting certain genes are risk factors for some types of cancer, but only smoking can be avoided. Risk factors that a person can control are called modifiable risk factors.

Many other factors in our environment, diet, and lifestyle may cause or prevent cancer. This summary reviews only the major cancer risk factors and protective factors that can be controlled or changed to reduce the risk of cancer. Risk factors that are not described in the summary include certain sexual behaviors, the use of estrogen, and being exposed to certain substances at work or to certain chemicals.

Factors That are Known to Increase the Risk of Cancer

Cigarette Smoking and Tobacco Use

Tobacco use is strongly linked to an increased risk for many kinds of cancer. Smoking cigarettes is the leading cause of the following types of cancer:

  • Acute myelogenous leukemia (AML).
  • Bladder cancer.
  • Cervical cancer.
  • Esophageal cancer.
  • Kidney cancer.
  • Lung cancer.
  • Oral cavity cancer.
  • Pancreatic cancer.
  • Stomach cancer.

Not smoking or quitting smoking lowers the risk of getting cancer and dying from cancer. Scientists believe that cigarette smoking causes about 30% of all cancer deaths in the United States.

See the following PDQ summaries for more information:

Infections

Certain viruses and bacteria are able to cause cancer. Viruses and other infection-causing agents cause more cases of cancer in the developing world (about 1 in 4 cases of cancer) than in developed nations (less than 1 in 10 cases of cancer). Examples of cancer-causing viruses and bacteria include:

Two vaccines to prevent infection by cancer-causing agents have already been developed and approved by the U.S. Food and Drug Administration (FDA). One is a vaccine to prevent infection with hepatitis B virus. The other protects against infection with strains of human papillomavirus (HPV) that cause cervical cancer. Scientists continue to work on vaccines against infections that cause cancer.

See the following PDQ summaries for more information:

Radiation

Being exposed to radiation is a known cause of cancer. There are two main types of radiation linked with an increased risk for cancer:

Scientists believe that ionizing radiation causes leukemiathyroid cancer, and breast cancer in women. Ionizing radiation may also be linked to myeloma and cancers of the lungstomachcolonesophagusbladder, and ovary. Being exposed to radiation from diagnostic x-rays increases the risk of cancer in patients and x-ray technicians. Diagnostic radiation in children and adolescents has been linked with a higher risk of cancers at a young age.

The growing use of CT scans over the last 20 years has increased exposure to ionizing radiation. The risk of cancer also increases with the number of CT scans a patient has and the radiation dose used each time.

See the following PDQ summaries for more information:

Immunosuppressive Medicines After Organ Transplant

Immunosuppressive medicines are used after an organ has been transplanted from one person to another. These medicines stop an organ that has been transplanted from being rejected. These medicines decrease the body’s immune response to help keep the organ from being rejected. Immunosuppressive medicines are linked to an increased risk of cancer because they lower the body’s ability to keep cancer from forming. The risk of cancer, especially cancer caused by a virus, is higher in the first 6 months after organ transplant, but the risk lasts for many years.

Factors That May Affect the Risk of Cancer

Diet

The foods that you eat on a regular basis make up your diet. Diet is being studied as a risk factor for cancer. It is hard to study the effects of diet on cancer because a person’s diet includes foods that may protect against cancer and foods that may increase the risk of cancer.

It is also hard for people who take part in the studies to keep track of what they eat over a long period of time. This may explain why studies have different results about how diet affects the risk of cancer.

Some studies have shown that a diet high in fat, proteinscalories, and red meat increases the risk of colorectal cancer, but other studies have not shown this.

It is not known if a diet low in fat and high in fiber, fruits, and vegetables lowers the risk of colorectal cancer.

See the following PDQ summaries for more information:

Alcohol

Studies have shown that drinking alcohol is linked to an increased risk of the following types of cancers:

Drinking alcohol may also increase the risk of liver cancer and female colorectal cancer.

See the following PDQ summaries for more information:

Physical Activity

Studies show that people who are physically active have a lower risk of certain cancers than those who are not. It is not known if physical activity itself is the reason for this.

Studies show a strong link between physical activity and a lower risk of colorectal cancer. Some studies show that physical activity protects against postmenopausal breast cancer and endometrial cancer.

See the following PDQ summaries for more information:

Obesity

Studies show that obesity is linked to a higher risk of the following types of cancer:

  • Postmenopausal breast cancer.
  • Colorectal cancer.
  • Endometrial cancer.
  • Esophageal cancer.
  • Kidney cancer.
  • Pancreatic cancer.

Some studies show that obesity is also a risk factor for cancer of the gallbladder and liver cancer.

It is not known if losing weight lowers the risk of cancers that have been linked to obesity.

See the following PDQ summaries for more information:

Diabetes

Some studies show that having diabetes may slightly increase the risk of having the following types of cancer:

  • Bladder cancer.
  • Breast cancer in women.
  • Colorectal cancer.
  • Endometrial cancer.
  • Liver cancer.
  • Lung cancer.
  • Oral cancer.
  • Oropharyngeal cancer.
  • Ovarian cancer.
  • Pancreatic cancer.

Diabetes and cancer share some of the same risk factors. These risk factors include the following:

  • Being older.
  • Being obese.
  • Smoking.
  • Not eating a healthy diet.
  • Not exercising.

Because diabetes and cancer share these risk factors, it is hard to know whether the risk of cancer is increased more by diabetes or by these risk factors.

Studies are being done to see how medicine that is used to treat diabetes affects cancer risk.

Environmental Risk Factors

Being exposed to chemicals and other substances in the environment has been linked to some cancers:

  • Links between air pollution and cancer risk have been found. These include links between lung cancer and secondhand tobacco smoke, outdoor air pollution, and asbestos.
  • Drinking water that contains a large amount of arsenic has been linked to skin, bladder, and lung cancers.

Studies have been done to see if pesticides and other pollutants increase the risk of cancer. The results of those studies have been unclear because other factors can change the results of the studies.

Interventions That are Known to Lower Cancer Risk

KEY POINTS

  • Chemoprevention is being studied in patients who have a high risk of developing cancer.

An intervention is a treatment or action taken to prevent or treat disease, or improve health in other ways. Many studies are being done to find ways to keep cancer from starting or recurring (coming back).

Chemoprevention is being studied in patients who have a high risk of developing cancer.

Chemoprevention is the use of substances to lower the risk of cancer, or keep it from recurring. The substances may be natural or made in the laboratory. Some chemopreventive agents are tested in people who are at high risk for a certain type of cancer. The risk may be because of a precancerous conditionfamily history, or lifestyle factors.

Taking one of the following agents may lower the risk of cancer:

See the NCI website for more information about cancer prevention.

Interventions That Are Not Known to Lower Cancer Risk

KEY POINTS

  • Vitamin and dietary supplements have not been shown to prevent cancer.
  • New ways to prevent cancer are being studied in clinical trials.

Vitamin and dietary supplements have not been shown to prevent cancer.

An intervention is a treatment or action taken to prevent or treat disease, or improve health in other ways.

There is not enough proof that taking multivitamin and mineral supplements or single vitamins or minerals can prevent cancer. The following vitamins and mineral supplements have been studied, but have not been shown to lower the risk of cancer:

The Selenium and Vitamin E Cancer Prevention Trial (SELECT) found that vitamin E taken alone increased the risk of prostate cancer. The risk continued even after the men stopped taking vitamin E. Taking selenium with vitamin E or taking selenium alone did not increase the risk of prostate cancer.

Vitamin D has also been studied to see if it has anticancer effects. Skin exposed to sunshine can make vitamin D. Vitamin D can also be consumed in the diet and in dietary supplements. Taking vitamin D in doses from 400-1100 IU/ day has not been shown to lower or increase the risk of cancer.

The VITamin D and OmegA-3 TriaL (VITAL) is under way to study whether taking vitamin D (2000 IU/ day) and omega-3 fatty acids from marine (oily fish) sources lowers the risk of cancer.

The Physicians’ Health Study found that men who have had cancer in the past and take a multivitamin daily may have a slightly lower risk of having a second cancer.

See the following PDQ summaries for more information:

New ways to prevent cancer are being studied in clinical trials.

Information about clinical trials supported by NCI can be found on NCI’s clinical trials search webpage. Clinical trials supported by other organizations can be found on the ClinicalTrials.gov website.

About This PDQ Summary

About PDQ

Physician Data Query (PDQ) is the National Cancer Institute’s (NCI’s) comprehensive cancer information database. The PDQ database contains summaries of the latest published information on cancer prevention, detection, genetics, treatment, supportive care, and complementary and alternative medicine. Most summaries come in two versions. The health professional versions have detailed information written in technical language. The patient versions are written in easy-to-understand, nontechnical language. Both versions have cancer information that is accurate and up to date and most versions are also available in Spanish.

PDQ is a service of the NCI. The NCI is part of the National Institutes of Health (NIH). NIH is the federal government’s center of biomedical research. The PDQ summaries are based on an independent review of the medical literature. They are not policy statements of the NCI or the NIH.

Purpose of This Summary

This PDQ cancer information summary has current information about cancer prevention. It is meant to inform and help patients, families, and caregivers. It does not give formal guidelines or recommendations for making decisions about health care.

Reviewers and Updates

Editorial Boards write the PDQ cancer information summaries and keep them up to date. These Boards are made up of experts in cancer treatment and other specialties related to cancer. The summaries are reviewed regularly and changes are made when there is new information. The date on each summary (“Updated”) is the date of the most recent change.

The information in this patient summary was taken from the health professional version, which is reviewed regularly and updated as needed, by the PDQ Screening and Prevention Editorial Board.

Clinical Trial Information

A clinical trial is a study to answer a scientific question, such as whether one treatment is better than another. Trials are based on past studies and what has been learned in the laboratory. Each trial answers certain scientific questions in order to find new and better ways to help cancer patients. During treatment clinical trials, information is collected about the effects of a new treatment and how well it works. If a clinical trial shows that a new treatment is better than one currently being used, the new treatment may become “standard.” Patients may want to think about taking part in a clinical trial. Some clinical trials are open only to patients who have not started treatment.

Clinical trials can be found online at NCI’s website. For more information, call the Cancer Information Service (CIS), NCI’s contact center, at 1-800-4-CANCER (1-800-422-6237).

PDQ® Screening and Prevention Editorial Board. PDQ Cancer Prevention Overview. Bethesda, MD: National Cancer Institute. Updated <02/20/2020>. Available at: https://www.cancer.gov/about-cancer/causes-prevention/patient-prevention-overview-pdq. Accessed <01/19/2021>. [PMID: 26389424]

 

Understanding Cancer Research Study Design and How to Evaluate Results

Photo of scientist doing cancer research SCOR

Approved by the Cancer.Net Editorial Board, 04/2018

Doctors and scientists conduct research studies to find better ways to prevent and treat cancer. Depending on the questions they want to answer, researchers can design these studies in different ways. No study design is perfect. Each has strengths and drawbacks. It is important to understand a study’s design. By doing this, you can understand the results to know if they apply to your situation.

In cancer research, there are 2 main types of research studies:

  • Experimental studies. This type of study provides an intervention, such as a new treatment. The intervention is given to a group of people. Then, researchers compare their results to those of another group that does not receive the intervention. This other group is known as the control group. The researchers choose who does and does not receive the intervention either randomly or through a selection process. Experimental studies help researchers learn more about how cancer starts or spreads. These studies can also test new imaging techniques and explore quality of life issues.
  • Observational studies. This type of study involves observing groups of people in a natural setting and looking at a specific result. A result may include whether 1 group of people has more cancer diagnoses than another group. In these studies, the researchers cannot control the intervention, such as a person’s weight or whether they took vitamin supplements. These studies are often described as epidemiologic. Epidemiology involves studying how different risks cause or spread a disease in a community.

Types of experimental studies

Experimental studies are more reliable than observational studies. This is because the volunteers are placed in the intervention or control group by chance. This reduces the likelihood that the assumptions or preferences of the researchers or volunteers will change the study results. Such assumptions or preferences are called bias.

This type of study also helps researchers to better find and control other factors, such as age, sex, and weight. These factors can affect the results of the study.

Researchers may also consider certain factors when choosing people to enroll in an experimental study. They could be based on type of cancer, stage of the disease, or whether the cancer has spread.

One of the most common types of experimental studies is the clinical trial. This is a research study that tests a medical intervention in people. Clinical trials test:

  • The effectiveness or safety of a new drug or combination of drugs
  • A new approach to radiation therapy or surgery
  • A new treatment or way to prevent cancer
  • Ways to lower the risk of cancer coming back

Doctors and researchers conduct clinical research in segments called phases. Each phase of a clinical trial provides different answers about the new treatment. For instance, it can show the dose, safety, and efficacy of the treatment. The efficacy is how well the treatment works. There are 4 phases of clinical trials.

In a clinical trial, volunteers are usually selected by chance to either be in the treatment or control group. Researchers can prevent bias in a clinical trial by keeping volunteers and/or themselves from knowing how the volunteers are grouped. This is a process known as “blinding.”

Types of experimental studies include:

  • Double-blind randomized trial. Most scientists believe this type of clinical trial will produce the best evidence in a study. Neither the volunteers nor the researchers know who belongs to a treatment or control group until the study ends.
  • Single-blind randomized trial. In this type of trial, the volunteers do not know whether they belong to a treatment or control group. But the researchers know.
  • Open/unblinded trial. Both volunteers and researchers know who belongs to each test group in this type of study. This occurs when it is not possible to use blinding. For instance, the study could compare a surgical treatment to a drug.

Types of observational studies

In observational studies, researchers have less control over the study volunteers. This means that certain factors could affect the results. These studies, however, are useful in providing initial evidence that can help guide future research.

Types of observational studies include:

  • Case-control studies. These types of studies compare 2 groups of people. For instance, they could compare those who have cancer (the case) and those who do not (the control). Researchers may look for lifestyle or genetic differences between the 2 groups. By doing this, they hope to find out why 1 group has a disease and the other group does not. These studies are conducted retrospectively. That is, they are researching what has already happened.
  • Cohort studies. These studies are prospective, which means that researchers study the event as it occurs. They monitor a group of people for a long time and track something. For example, they could track any new cancer diagnoses. This type of study can assess whether certain nutrients or actions can prevent cancer. This approach can also find cancer risk factors. For instance, cohort studies have looked at whether postmenopausal hormone replacement therapy increases the risk of breast cancer.
  • Case reports and case series. These studies are detailed descriptions of a patient’s medical history. The individual patient descriptions are called case reports. If many patients receive a similar treatment, the case reports may be combined into a case series. The results of case series studies are descriptions of patients’ histories within a specific group. As such, they should not be used to determine treatment options.
  • Cross-sectional studies. These studies examine how diseases interact with other factors within a specific group at a point in time. But because these studies only measure interactions at a single point in time, they cannot prove that something causes cancer.

Types of review articles

A large number of cancer research studies are published every year. Given this, it is challenging for doctors, as well as interested patients and caregivers, to keep up with the latest advances. Research studies published in journals are constantly shaping and reshaping the scientific understanding of that subject. No single study provides the final word on a topic, type of cancer, or treatment. As a result, review articles, which evaluate and summarize the findings of all published research on a certain topic, are extremely helpful.

Types of review articles include:

  • Systematic reviews. These articles summarize the best available research on a specific topic. Researchers use an organized method to locate, gather, and evaluate a number of research studies on a particular topic. By combining the findings of a number of studies, researchers are able to draw more reliable conclusions.
  • Meta-analyses. These studies combine data from several research studies on the same topic. By combining these data, a meta-analysis can find trends that are hard to see in smaller studies. But if the single studies were poorly designed, the results of the meta-analysis may not be useful.

Evaluating research studies

Here are some tips for finding out the quality of a research study:

  • Find out if the journal uses a peer-review process. Results from a study are more reliable if they are peer-reviewed. This means that researchers who are not a part of the study have looked over and approved the design and methods.
  • Look at the length of the study and the number of people involved. A study is more useful and credible if the same results occur in many people across a long time. Studies of rare types of cancer or cancers with a poor chance of getting better are an exception to this rule. This is because there are a small number of patients to study. Also, when looking at the length of the study, it may be suitable for some clinical trials to be shorter. For instance, cancer prevention trials are often much longer than treatment clinical trials. This is because it usually takes longer to figure out if a prevention strategy is working compared to a treatment.
  • Consider the phase of the study when learning about new treatments. Phase I and II clinical trials usually tell you more about the safety of a treatment and less about how well it works. These studies tend to have a smaller number of patients compared to phase III clinical trials.  Phase III clinical trials compare a new treatment with the standard of care. “Standard of care” means the best treatments known. Doctors consider phase III clinical trials to be the most reliable.
  • Find out if the study supports or contradicts current research. New results are exciting, but other researchers must validate the results before the medical field accepts them as fact. Review articles like systematic reviews are of special interest. They review and draw conclusions across all of the published research on a specific topic.
  • Watch out for conclusions that overstate or oversimplify the results. Each study is a small piece of the research puzzle. Medical practice rarely changes because of the results of a single study.

Questions to ask your health care team

Always talk to your health care team about what you find in an abstract or study. If you have reviewed a study that suggests a different approach to cancer treatment, do not stop or change your treatment. First talk with your health care team about how the study relates to your treatment plan.

Consider asking your health care team the following questions:

  • I recently heard about a study that used a new treatment. Is this treatment related to my type and stage of cancer?
  • What type of journals should I read to learn more about my type of cancer?
  • Should I consider being a part of a clinical trial?
  • What clinical trials are open to me?
  • Where can I learn more about clinical trials?