Cancer Screening: An Overview

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, a cancer may have grown and spread, which can make it harder to treat.

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

In short: screening looks for cancer early, before you feel anything wrong.

Screening tests include:

• Physical exam and history — a check of the body for general signs of health, including lumps or anything unusual, plus a look at your health habits and past illnesses.
• Laboratory tests — tests of samples such as tissue, blood, or urine.
• Imaging procedures — procedures that make pictures of areas inside the body.
• Genetic tests — a lab test that looks for changes in genes or chromosomes that may be a sign of a disease or a risk of one.

Not all screening tests are helpful, and most have risks. It helps to know a test's risks and whether it has been shown to lower the chance of dying from cancer.

• Some tests can cause problems. For example, colon cancer screening with sigmoidoscopy or colonoscopy can cause tears in the lining of the colon.
• False-positive results are possible. A test may appear abnormal even when there is no cancer. This can cause anxiety and usually leads to more tests, which also have risks.
• False-negative results are possible. A test may appear normal even when cancer is present. This may cause a person to delay care.
• Finding a cancer may not help. Some cancers never cause symptoms or become life-threatening. There is no way to know if treating such a cancer would help a person live longer, and treatments have side effects.

In short: every screening test has benefits and harms.

Understanding the benefits and harms of screening tests is important for choosing which ones are right for you. Before having any screening test, discuss it with your doctor or other healthcare provider. Your provider should talk with you about the benefits, harms, and unknowns of a test and include you in the decision. This is called informed and shared decision-making. Sometimes the harms and benefits are closely matched, and the choice is hard to make.

A screening test that works well finds cancer before symptoms appear, screens for a cancer that is easier to treat when found early, has few false results, and lowers the chance of dying from that cancer.

Screening tests are not meant to diagnose cancer. If a result is abnormal, more tests may be done to check for cancer. For example, a screening mammogram may find a lump, but a lump may be cancer or something else. Diagnostic tests, which may include a biopsy, are needed to find out.

For many cancers, the chance of recovery depends on the stage — how much the cancer has spread — when it is found. Cancers found at earlier stages are often easier to treat. Some screening tests have been shown both to find cancers early and to lower the chance of dying from them, such as mammograms for breast cancer, and sigmoidoscopy and fecal occult blood testing for colorectal cancer. Other tests find cancer early but have not been proven to lower the risk of dying from it.

Studies of screening compare death rates in people who are screened with those who are not. Some things can make screening look more helpful than it is, such as lead-time bias, overdiagnosis, and healthy screenee bias. This is why researchers study screening carefully before a test becomes a standard test.

Your healthcare team is the best source of information about which screening tests make sense for you.