Cancer screening is looking for cancer before a person has any symptoms.

Screening tests can help find cancer at an early stage, before symptoms appear. When abnormal tissue or cancer is found early, it may be easier to treat or cure. By the time symptoms appear, the cancer may have grown and spread. This can make the cancer harder to treat or cure.

It is important to remember that when your doctor suggests a screening test, it does not always mean he or she thinks you have cancer. Screening tests are done when you have no cancer symptoms.

There are different kinds of screening tests.

Screening tests include the following:

• Physical exam and history: An exam of the body to check general signs of health, including checking for signs of disease, such as lumps or anything else that seems unusual. A history of the patient's health habits and past illnesses and treatments will also be taken.
• Laboratory tests: Medical procedures that test samples of tissue, blood, urine, or other substances in the body.
• Imaging procedures: Procedures that make pictures of areas inside the body.
• Genetic tests: A laboratory test in which cells or tissue are analyzed to look for changes in genes or chromosomes. These changes may be a sign that a person has or is at risk of having a specific disease or condition.

Screening tests have risks.

Not all screening tests are helpful and most have risks. It is important to know the risks of the test and whether it has been proven to decrease the chance of dying from cancer.

Understanding the benefits and harms of screening tests is important for making an informed choice about which screening tests are right for you.

Before having any screening test, discuss the test with your doctor or other health care provider. Every screening test has both benefits and harms. Your health care provider should talk to you about the benefits and harms of a screening test and include you in the decision about whether the screening test is right for you. This is called informed and shared decision-making.

1. Your health care provider will talk to you about the possible benefits, harms, and unknowns of a screening test. This may include information about the benefits of finding a cancer early or the harms related to false test results, overdiagnosis, and overtreatment. Your health care provider may also give you information in a leaflet, booklet, video, website, or other material.
2. After you understand the benefits and harms of a screening test, you can decide whether or not you want to have the screening test based on what is best for you. Sometimes the harms and benefits are closely matched and the decision about whether to have a screening test is hard to make.
3. Your health care provider will write your decision down in your medical record and order the screening test, if that was your decision.

Screening tests have many goals.

A screening test that works the way it should and is helpful does the following:

• Finds cancer before symptoms appear.
• Screens for a cancer that is easier to treat and cure when found early.
• Has few false-negative test results and false-positive test results.
• Decreases the chance of dying from cancer.

Screening tests are not meant to diagnose cancer.

Screening tests usually do not diagnose cancer. If a screening test result is abnormal, more tests may be done to check for cancer. For example, a screening mammogram may find a lump in the breast. A lump may be cancer or something else. More tests need to be done to find out if the lump is cancer. These are called diagnostic tests. Diagnostic tests may include a biopsy, in which cells or tissues are removed so a pathologist can check them under a microscope for signs of cancer.

Certain screening tests may be suggested only for people who have a high risk for certain cancers.

Anything that increases the chance of cancer is called a cancer risk factor. Not every person with one or more risk factors will develop cancer, and it will develop in some people who don't have any known risk factors.

Some screening tests are used only for people who have known risk factors for certain types of cancer. People known to have a higher risk of cancer than others include those who have any of the following:

• A personal history of cancer.
• A family history of cancer.
• Certain gene mutations (changes) that have been linked to cancer.
• Exposure to cancer-causing agents such as tobacco smoke or workplace chemicals.
• A blood clot that develops for no known reason.
• Older age.

People who have a high risk of cancer may need to be screened more often or at an earlier age than other people.

Cancer screening research includes finding out who has an increased risk of cancer.

Scientists are trying to better understand who is likely to get certain types of cancer. They study the things we do and the things around us to see if they cause cancer. This information helps doctors figure out who should be screened for cancer, which screening tests should be used, and how often the tests should be done.

Since 1973, the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute has been collecting information on people with cancer from different parts of the United States. Information from SEER, research studies, and other sources is used to study who is at risk.

Finding some cancers at an early stage (before symptoms appear) may help decrease the chance of dying from those cancers.

For many cancers, the chance of recovery depends on the stage (the amount or spread of cancer in the body) of the cancer when it was diagnosed. Cancers that are diagnosed at earlier stages are often easier to treat or cure.

Studies of cancer screening compare the death rate of people screened for a certain cancer with the death rate from that cancer in people who were not screened. Some screening tests have been shown to be helpful both in finding cancers early and in decreasing the chance of dying from those cancers. These include mammograms for breast cancer and sigmoidoscopy and fecal occult blood testing for colorectal cancer. Other tests are used because they have been shown to find a certain type of cancer in some people before symptoms appear, but they have not been proven to decrease the risk of dying from that cancer. If a cancer is fast-growing and spreads quickly, finding it early may not help the person survive the cancer.

Screening studies are done to see whether deaths from cancer decrease when people are screened.

When collecting information on how long people with cancer live, some studies define survival as living 5 years after the diagnosis. This is often used to measure how well cancer treatments work. However, to see if screening tests are useful, studies usually look at whether deaths from the cancer decrease in people who were screened. Over time, signs that a cancer screening test is working include:

• An increase in the number of early-stage cancers found.
• A decrease in the number of late-stage cancers found.
• A decrease in the number of deaths from the cancer.

The number of deaths from cancer is lower today than it was in the past. It is not always clear if this is because screening tests found the cancers earlier or because cancer treatments have gotten better, or both. The Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute collects and reports information on survival times of people with cancer in the United States. This information is studied to see if finding cancer early affects how long these people live.

Certain factors may cause survival times to look like they are getting better when they are not.

These factors include lead-time bias, overdiagnosis, and healthy screenee bias.

• Lead-time bias
  Survival time for people with cancer is usually measured from the day the cancer is diagnosed until the day they die. People are often diagnosed after they have signs and symptoms of cancer. If a screening test leads to a diagnosis before a person has any symptoms, the person's survival time is increased because the date of diagnosis is earlier. This increase in survival time makes it seem as though screening helps people live longer when that may not be happening. This is called lead-time bias. Survival time may appear to be longer because the date of diagnosis is earlier for the people who are screened, but they may die at the same time they would have without the screening test.
• Overdiagnosis
  Sometimes, screening tests find cancers that don't matter because they would have gone away on their own or never caused any symptoms. These cancers would never have been found if not for the screening test (for example, a computed tomography scan). Finding these cancers is called overdiagnosis. Overdiagnosis can make it seem like more people are surviving cancer longer, but in reality, these are people who would not have died from cancer anyway.
• Healthy screenee bias
  People who choose to have screening tests are often more health conscious than the general public and tend to make other healthy choices in their lives. People with early-stage cancer found by screening may live longer than people who don't get screened. They may live longer because of the benefits of their healthy lifestyle rather than screening. This is sometimes called healthy screenee bias.

Results from research studies help doctors decide when a screening test works well enough to be used as a standard test.

Evidence about how safe, accurate, and useful cancer screening tests are comes from clinical trials (research studies with people) and other kinds of research studies. When enough evidence has been collected to show that a screening test is safe, accurate, and useful, it becomes a standard test. Examples of cancer screening tests that were once under study but are now standard tests include:

• Colonoscopy for colorectal cancer.
• Mammograms for breast cancer.
• Pap tests (Pap smears) for cervical cancer.

Different types of research studies are done to study cancer screening.

Cancer screening trials study new ways of finding cancer in people before they have symptoms. Screening trials also study screening tests that may find cancer earlier or are more accurate than existing tests, or that may be easier, safer, or cheaper to use. Screening trials are designed to find the possible benefits and possible harms of cancer screening tests. Different clinical trial designs are used to study cancer screening tests.

The strongest evidence about screening comes from research done in clinical trials. However, clinical trials cannot always be used to study questions about screening. Findings from other types of studies can give useful information about how safe, useful, and accurate cancer screening tests are.

The following types of studies are used to get information about cancer screening tests:

Screening tests for 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.

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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.

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Purpose of This Summary

This PDQ cancer information summary has current information about cancer screening. 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).

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PDQ® Screening and Prevention Editorial Board. PDQ Cancer Screening Overview. Bethesda, MD: National Cancer Institute. Updated <MM/DD/YYYY>. Available at: https://www.cancer.gov/about-cancer/screening/patient-screening-overview-pdq. Accessed <MM/DD/YYYY>. [PMID: 26389447]

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