Cancer Screening, Explained

What happens

• Screening means testing people who have no symptoms, to find cancer early, when treatment often works best.
• Some screening finds pre-cancer, like bowel polyps or abnormal cervical cells, so it can be treated before it becomes cancer.
• The main proven programs are for bowel (colorectal), breast, cervical and, for some people with a long history of heavy smoking, lung cancer.
• Screening has trade-offs, including false alarms and overdiagnosis. Ages and timing vary by place and change over time, so ask your clinician or local screening program.
• Screening is not for symptoms. A new lump, unusual bleeding, unexplained weight loss or a cough that won't go away should be checked now.

Know the warning signs

• A new lump or swelling
• Unusual bleeding
• Weight loss you can't explain
• A cough that won't go away

These are often caused by something less serious than cancer, but don't wait for a screening test. See a doctor or nurse practitioner.

Call 9-1-1 or your local emergency number.

What you can do

• Ask your clinician or local screening program which screening tests are right for you, and when.
• Don't smoke. If you smoke, getting help to quit lowers your risk of many cancers.
• Get the HPV vaccine if it's recommended for you. It protects against the virus behind most cervical cancers.
• Protect your skin from the sun, and with alcohol, less is better.
• Stay active and aim for a healthy weight.

Common questions

At what age should I start cancer screening?

It depends on the test and on where you live, and recommendations change over time. In many places bowel screening starts around age 45 to 50, many breast screening programs start in your 40s or at 50, and cervical screening usually starts in your 20s. Lung screening is only for some older adults with a long history of heavy smoking. Your clinician or local screening program can tell you what applies to you.

What is the difference between screening and a test for symptoms?

Screening is for people who feel well and have no symptoms. If you have a symptom, such as a new lump, unusual bleeding, weight loss you can't explain or a cough that won't go away, you need a check-up with a doctor or nurse practitioner now, not a screening appointment.

Can cancer screening do harm?

Sometimes. A test can flag something that turns out to be nothing (a false alarm), which can mean worry and more tests. Screening can also find slow-growing cancers that would never have caused harm, which is called overdiagnosis. That's why screening is recommended only for certain cancers and certain people, and why some tests, like the PSA test for prostate cancer, are a choice to talk through with your clinician.

Transcript

Some of the most useful health tests are the ones you take when you feel perfectly fine.

They're called cancer screening tests. They look for trouble early, sometimes before it has even turned into cancer. Let's see how screening works, and how to make it work for you.

Your body is made of trillions of cells. Every day, old cells wear out, and new ones replace them, in careful, controlled steps.

Cancer starts when some cells stop following those rules. They grow and divide out of control, and can form a lump called a tumour. Over time, they can invade nearby tissue and spread.

Many cancers grow slowly, over years. Early on, they're often small, and cause no symptoms at all.

But that quiet early stage is often when treatment works best. Found early, many cancers are smaller, easier to treat, and more likely to be cured.

That's the idea behind screening: testing people who have no symptoms, to find cancer early.

For some cancers, screening can do even better. It can find changes that haven't become cancer yet, called pre-cancer, and treat them before they ever do.

In the bowel, most cancers begin as a small growth called a polyp. Removing polyps during a colonoscopy can stop cancer before it starts.

On the cervix, screening can find abnormal cells, usually caused by a common virus called HPV. Treating those cells can prevent cervical cancer.

So which screening tests are proven to help? There are four main ones.

Colorectal, or bowel, cancer. You can do a simple stool test at home, often called FIT, or have a colonoscopy. In many places, it starts somewhere around age forty-five to fifty.

Breast cancer, with a mammogram, a low-dose X-ray of the breast. Many programs start in your forties, or at fifty.

Cervical cancer, with an HPV test or a Pap test, usually starting in your twenties.

And lung cancer, with a low-dose CT scan, for some older adults with a long history of heavy smoking, including some who have quit.

The exact ages, and how often to test, differ by country, province and state, and they change as new evidence comes in. Your clinician, or your local screening program, can tell you what's right for you.

Screening isn't perfect, though. Sometimes a test flags something that turns out to be nothing. That's a false alarm, and it can mean worry, and more tests.

Screening can also find some cancers that grow so slowly they would never have caused harm. This is called overdiagnosis, and it can lead to treatment a person didn't need.

That's why screening isn't recommended for every cancer, or for everyone. The PSA blood test for prostate cancer, for example, has both benefits and harms. Guidelines differ, so it's a choice to make with your clinician.

One more key point: screening is for people without symptoms. If you notice a new lump, unusual bleeding, weight loss you can't explain, or a cough that won't go away, don't wait for a screening test.

Get it checked by a doctor or nurse practitioner now. Most of the time it's something less serious, but it's worth finding out.

Screening works best alongside prevention. The Canadian Cancer Society estimates that about four in ten cancer cases could be prevented.

The biggest step is not smoking. If you smoke, getting help to quit lowers your risk of many cancers.

The HPV vaccine protects against the virus behind most cervical cancers. Protect your skin from the sun. And with alcohol, less is better.

Stay active, and aim for a healthy weight. Small changes add up.

One common myth: "I feel fine, and no one in my family had cancer, so I don't need screening." In fact, most cancers aren't inherited, and the biggest risk is simply getting older. Early cancer often feels like nothing at all.

So, to recap. Screening looks for trouble before symptoms. It can stop some cancers before they start. It has trade-offs worth discussing. And symptoms mean: get checked now.

Now you know how cancer screening works. Ask your clinician which tests are right for you, and when.

Sources

1. U.S. Preventive Services Task Force — Colorectal Cancer: Screening (2021)
2. U.S. Preventive Services Task Force — Breast Cancer: Screening (2024)
3. U.S. Preventive Services Task Force — Prostate Cancer: Screening (2018)
4. Canadian Task Force on Preventive Health Care — screening guidelines
5. Canadian Cancer Society — Find cancer early
6. Canadian Cancer Society — Prevention statistics (ComPARe)
7. American Cancer Society — Guidelines for the Early Detection of Cancer
8. WHO — Cervical cancer fact sheet