Cancer screening

Cancer screening is designed to identify precancerous changes or early cancer in people who do not have symptoms. Screening differs from diagnostic testing, which investigates a symptom, an abnormal examination, or a prior test result.¹ An effective program must balance the chance of finding important disease against false-positive results, unnecessary procedures, complications, and overdiagnosis—the detection of a cancer that would never have caused harm during a person’s lifetime.

Screening is most useful when a cancer is sufficiently common in the target population, has a detectable early stage, and has treatment that improves outcomes when started earlier. Recommendations therefore specify age ranges, intervals, risk groups, and the circumstances in which screening should stop. Personal and family history, inherited risk, previous treatment, and tobacco exposure can alter those recommendations.

Screening programs target a limited number of cancers for which evidence supports a favorable balance of benefits and harms. Common examples include mammography for breast cancer, cervical cytology or testing for high-risk human papillomavirus for cervical cancer, stool-based tests or colonoscopy for colorectal cancer, and low-dose computed tomography for lung cancer in adults at substantial smoking-related risk.²³⁴

These methods do not all search for cancer in the same way. Cervical screening can identify precancer before invasion, while colorectal screening may find and permit removal of precancerous polyps. Lung screening uses repeated imaging and is restricted to carefully defined smoking-risk groups. There is no generally recommended population screening test for every cancer; many blood tests, scans, and tumor-marker panels remain investigational or are used only in selected high-risk settings.

A screening benefit is measured by outcomes such as fewer deaths or fewer advanced cancers, not simply by finding more tumors. Earlier detection can improve treatment options, but apparent survival gains can also arise from lead-time bias, length bias, and overdiagnosis.¹ For that reason, randomized trials and long-term population evidence are used to assess screening programs.

Abnormal results require a diagnostic pathway rather than immediate assumptions about cancer. Depending on the test, follow-up may include repeat testing, imaging, endoscopy, or biopsy. False positives can cause anxiety, expense, radiation exposure, or invasive procedures; false negatives can delay reassurance or diagnosis. Shared decision-making is especially relevant when benefits are modest, harms are substantial, or recommendations vary by age and risk.

Screening eligibility often depends more on risk history than on age alone. People with a strong family history, hereditary cancer syndrome, prior precancer, or previous radiation exposure may need earlier, more intensive, or specialized surveillance, while average-risk recommendations may not apply to them.

Screening programs also depend on access and follow-through: an accurate test provides little population benefit when invitations, transportation, laboratory capacity, communication, or diagnostic follow-up are inadequate. Cervical screening is a notable prevention model because vaccination against human papillomavirus reduces risk but does not eliminate the need for screening. Conversely, some widely marketed multi-cancer blood tests can detect signals associated with cancer, but their effects on cancer mortality and the balance of downstream harms are not yet established for routine population use.¹

Diagnostic testing

Testing performed to investigate symptoms, signs, or an abnormal screening result.

False positive

A screening result suggesting possible disease when the person does not have the target cancer or precancer.

Overdiagnosis

Detection of a cancer that would not have caused symptoms or death during the person’s lifetime.

Precancer

An abnormal cellular change that may progress to invasive cancer but is not yet cancer.

Risk-based screening

Screening tailored to factors such as age, smoking exposure, family history, or inherited susceptibility.