A mammogram is an X-ray of the breast that is used to look for breast cancer.

There are two types of mammograms.

• A screening mammogram looks for signs of breast cancer before any symptoms appear.
• A diagnostic mammogram looks for breast cancer in women who have symptoms. Or it is done if you had an abnormal screening mammogram. It's also used to find an area of suspicious breast tissue to remove for a biopsy.

This decision aid is about screening mammograms.

Mammograms can find some breast cancers early, when the cancer may be more easily treated. Often a mammogram can find cancers that are too small for you or your doctor to feel.

Studies show that a small number of women who have mammograms may be less likely to die from breast cancer.

The risk for breast cancer goes up as you get older. In general, women younger than 50 are at a lower risk for breast cancer. Because of this, women ages 50 to 70 are more likely to benefit from having a mammogram than women who are in their 40s.

Each time you have a mammogram, there is a risk that the test:

• May miss some breast cancers. And some cancers that are found may still be fatal, even with treatment.
• May show an abnormal result when it turns out there wasn't any cancer (called a false-positive). This means you may need more tests—such as another mammogram, a breast ultrasound, or a biopsy—to make sure you don't have cancer. These tests can be harmful and cause a lot of worry.
• May find cancers that will never cause a problem (called overdiagnosis). Some breast cancers never grow or spread and are harmless. You might have this type of cancer, but a mammogram can't tell whether it's harmless. So you may get cancer treatment—including surgery, radiation, or chemotherapy—that you don't need.
• Will briefly expose you to very small amounts of radiation. While the risk from being exposed to radiation from a mammogram is low, it can add up over time.

Benefits

*Based on the best available evidence (evidence quality: moderate to high)

*Based on the best available evidence (evidence quality: moderate to high)

Breast cancer diagnosis

Mammograms can find some breast cancers early, when the cancer may be more easily treated. Often a mammogram can find cancers that are too small for you or your doctor to feel.

Take a group of women who have a mammogram every year for 10 years.^{footnote 3}

• About 19 out of 1,000 women ages 40 to 49 will be diagnosed with breast cancer. This means that about 981 won't.
• About 30 out of 1,000 women ages 50 to 59 will be diagnosed with breast cancer. This means that about 970 won't.
• About 44 out of 1,000 women ages 60 to 69 will be diagnosed with breast cancer. This means that about 956 won't.

Survival

Studies show that a small number of women who have mammograms may be less likely to die from breast cancer. Since the risk for breast cancer goes up as you get older, women ages 50 to 70 are more likely to benefit from having a mammogram than women who are in their 40s.

Take a group of women who have a mammogram every year for 10 years.^{footnote 3}

• Mammograms will make a life-saving difference for less than 1 out of 1,000 women ages 40 to 49. This means that for more than 999 of them, having mammograms won't make a difference.
• Mammograms will make a life-saving difference for about 1 out of 1,000 women ages 50 to 59. This means that for about 999 of them, having mammograms won't make a difference.
• Mammograms will make a life-saving difference for about 4 out of 1,000 women ages 60 to 69. This means that for about 996 of them, having mammograms won't make a difference.

The numbers in the tables and narratives include women who are expected to be diagnosed with invasive breast cancer or an early form of noninvasive breast cancer called ductal carcinoma in situ.

Risks

*Based on the best available evidence (evidence quality: moderate to high)

Mammograms may show an abnormal result when it turns out there wasn't any cancer (called a false-positive). This means you may need more tests—such as another mammogram, a breast ultrasound, or a biopsy—to make sure you don't have cancer. These tests can be harmful and cause a lot of worry.

Take a group of women who have a mammogram every year for 10 years.^{footnote 3}

• About 613 out of 1,000 women ages 40 to 49 will have at least one false-positive test result. This means that about 387 won't.
• About 613 out of 1,000 women ages 50 to 59 will have at least one false-positive test result. This means that about 387 won't.
• About 497 out of 1,000 women ages 60 to 69 will have at least one false-positive test result. This means that about 503 won't.

Breast cancer screening can also lead to unnecessary biopsies.

Take a group of women who have a mammogram every year for 10 years.^{footnote 3}

• About 70 out of 1,000 women ages 40 to 49 will have a biopsy they don't need. This means that about 930 won't.
• About 94 out of 1,000 women ages 50 to 59 will have a biopsy they don't need. This means that about 906 won't.
• About 98 out of 1,000 women ages 60 to 69 will have a biopsy they don't need. This means that about 902 won't.

Mammograms may find cancers that would never have caused a problem if they had not been found through screening (called overdiagnosis). But a mammogram can't tell whether the cancer is harmless. So you may get cancer treatment that you don't need.

Out of 1,000 women who are diagnosed with breast cancer during screening mammograms, experts estimate that about 190 of them may get treatment they don't need.^{footnote 3}

The numbers in the tables and narratives include women who are expected to be diagnosed with invasive breast cancer or an early form of noninvasive breast cancer called ductal carcinoma in situ.

Understanding the evidence

Some evidence is better than other evidence. Evidence comes from studies that look at how well treatments and tests work and how safe they are. For many reasons, some studies are more reliable than others. The better the evidence is—the higher its quality—the more we can trust it. The information shown here is based on the best available evidence.^{footnote 3} The evidence is rated using four quality levels: high, moderate, borderline, and inconclusive.

Another thing to understand is that the evidence can't predict what's going to happen in your case. When evidence tells us that 2 out of 100 people who have a certain test or treatment may have a certain result and that 98 out of 100 may not, there's no way to know if you will be one of the 2 or one of the 98.

For women who are at average risk for breast cancer, there are no easy answers for when to start having mammograms. Recommendations for when to start having mammograms vary from province to province. Talk to your doctor about what is right for you.

For example:

For women who are at average risk for breast cancer, the Canadian Task Force on Preventive Health Care recommends the following guidelines.^{footnote 1}

• Ages 40 to 49: Regular mammograms are not recommended.
• Ages 50 to 74: Regular mammograms (every 2 to 3 years) are recommended.
• Age 75 and older: You may want to talk to your doctor about whether you need breast cancer screening.

Most experts agree that all women should be informed about the risks and benefits of mammograms and offered screening by age 50.

When to stop having mammograms is another decision. You and your doctor will decide on the right age to stop screening based on your personal preferences and overall heath.