Cancer screening is often presented as a straightforward concept: find cancer earlier so it can be treated earlier. But the reality is more complex.

In this Masterclass, Dr. Penelope Harris examines the benefits and limitations of screening asymptomatic people for cancer. Drawing from her experience caring for cancer patients in military, academic, private-practice, and GoldCare settings, she explains why screening decisions should consider not only whether a test can detect cancer, but also a person’s individual risk, the accuracy of the test, and what happens after a result comes back positive or negative.

Why Do We Screen for Cancer?

The primary goal of cancer screening is to detect certain cancers before symptoms appear, ideally at a stage when treatment may be more effective.

But finding more cancer does not automatically mean preventing more deaths. Dr. Harris introduces this distinction and explains why evaluating screening requires looking at outcomes, not simply the number of cancers detected.

The Benefits—and Risks—of Screening

Screening can identify cancers earlier, but it can also lead to false positives, false negatives, incidental findings, additional imaging, biopsies, anxiety, cost, and potential overdiagnosis.

Overdiagnosis occurs when screening identifies a cancer that might never have caused significant harm during a person’s lifetime. Dr. Harris discusses why this is particularly relevant when considering certain slower-growing cancers and why the potential consequences of treatment should also be part of the conversation.

Understanding Screening Bias

The Masterclass also introduces concepts such as lead-time bias and length-time bias.

A cancer discovered earlier may make it appear that a patient survived longer after diagnosis even when the timing of death remains unchanged. Screening may also be more likely to identify slower-growing cancers simply because they remain detectable for longer periods.

Understanding these concepts helps explain why screening statistics can sometimes be more complicated than they initially appear.

How Are Different Cancers Screened?

Dr. Harris reviews several cancers for which screening is commonly recommended and discusses the methods used to detect them.

The presentation covers breast cancer screening with mammography, cervical cancer screening with Pap testing, colorectal cancer screening with colonoscopy and stool-based tests, prostate cancer screening with PSA testing, and lung cancer screening for certain higher-risk individuals.

She also discusses other tools used in specific circumstances, including ultrasound, MRI, PET and CT imaging, skin examinations, tumor markers, and additional screening for people with certain hereditary cancer syndromes.

Why Individual Risk Matters

Not every person has the same likelihood of developing cancer.

Age, family history, hereditary cancer syndromes, tobacco and alcohol exposure, obesity, physical inactivity, certain infections, environmental exposures, and other individual factors can affect risk.

Because of these differences, Dr. Harris emphasizes an individualized approach rather than assuming that every screening test provides the same value to every person.

The New Generation of Multi-Cancer Blood Tests

One of the newest developments in cancer screening is the emergence of multi-cancer early detection blood tests, including Galleri.

These tests look for biological signals associated with multiple cancers from a single blood sample. Dr. Harris reviews data presented for Galleri and discusses both its potential and its current limitations, including false-positive and false-negative results, cost, and the need for additional testing following a positive result.

She also briefly discusses other approaches, including RGCC testing, and explains how different tests may examine circulating DNA, RNA, proteins, methylation patterns, or other cancer-associated signals.

What We Still Don’t Know

Emerging cancer-detection technologies may expand the ability to identify cancers for which routine screening does not currently exist. However, Dr. Harris emphasizes an important unanswered question: Does detecting more cancers ultimately reduce cancer-related mortality and improve overall outcomes?

For newer multi-cancer screening tests, long-term mortality data are still developing. This distinction is important when evaluating what a new technology can detect versus what has been demonstrated to improve patient outcomes.

Making an Informed Decision

There is no single screening strategy that is appropriate for every person.

The potential benefit of finding cancer earlier must be considered alongside the possibility of false results, unnecessary procedures, overdiagnosis, cost, radiation exposure from follow-up imaging, personal risk factors, and family history.

The goal is not simply to screen more—it is to understand the available information, consider your individual circumstances, and make informed decisions with a trusted clinician.

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