Mammography is an x-ray of the breast that uses low doses of radiation. The picture made during mammography is called a mammogram. It can help find both cancerous (malignant) and non-cancerous (benign) tumours in the breast.

Why mammography is done

There are 2 types of mammography. They are done for different reasons.

Screening mammography is used to look for cancer in women who do not have any symptoms of breast cancer or breast problems. Both breasts are examined during a screening mammography. It can help find lumps or abnormal areas of breast tissue that may be too small to be felt by hand. Screening mammography monitors changes to the breast over time and can help find breast cancer at an early stage.

Diagnostic mammography is done to diagnose a breast problem, such as a lump or suspicious area. You may notice the problem and tell your doctor about it, or it may be found during a clinical breast examination or screening mammography. Diagnostic mammography takes longer than a screening mammography. More detailed images and views of the breast are taken from different angles, to look more closely at an area in the breast. Diagnostic mammography is usually done on both breasts so that doctors can compare the breast tissue of both breasts.

Diagnostic mammography may also be done to help find the abnormal area during a stereotactic core needle biopsy or wire localization biopsy.

Preparing for mammography

Don’t use deodorant, antiperspirant, body lotion or talcum powder under the arms or on the breasts before mammography. These products can interfere with the accuracy of the x-ray. You may be given a disposable towel to wipe down your skin around your breasts and armpits.

Wear clothing that is easy to remove from the waist up.

Remove necklaces and other jewellery that might get in the way during the test.

If you’re worried about discomfort during the test, these tips may help:

  • Try to avoid having mammography in the 7 to 10 days before your menstrual period begins. Your breasts may be tender during this time.
  • Avoid drinks and food that contain caffeine, such as coffee, tea, cola and chocolate, for 5–7 days before mammography.
  • Talk to your doctor about taking a mild pain medicine about an hour before your test.

How mammography is done

Mammography is done in clinics or hospital x-ray departments.

You will stand in front of the mammography machine, and your breast is placed between 2 plastic compression plates. The plates are then pressed together to flatten, or compress, the breast.

The breast tissue is compressed to make the mammogram clearer so the tissues inside the breast can be seen. Flattening the breast also allows less radiation to be used.

If you have a lot of discomfort during the x-ray, tell the radiation technologist who is doing the test. They may be able to adjust the compression to make it more comfortable.

Views of each breast are taken from different angles. Diagnostic mammography takes more views than screening mammography.

Before you get dressed, the technologist will check the mammography films (mammograms) to make sure they are clear enough for the radiologist (a doctor who specializes in using imaging techniques) to read. You may have to do the test again if the mammograms are not clear.

Spot compression

Spot compression (also called cone compression) may be used to get a closer view of one area of the breast during diagnostic mammography.

To get a clearer image, a small compression plate separates the breast tissue in an area and pushes normal breast tissue out of the way. The images may be enlarged (magnified) to make it easier to see small suspicious areas.

These close-up views can show tiny deposits of calcium (called calcifications) that look like small white spots on a mammogram. The radiologist looks at the size, shape and pattern of calcifications and reports these as part of the results of the mammography.

Most breast calcifications are not related to cancer. However, certain characteristics of calcifications, such as irregular shapes or certain groupings, can be a concern.

What the results mean

The radiologist will read the mammogram to look for changes or abnormalities. Many mammography centres in Canada use the American College of Radiologists Breast Imaging Reporting and Data System (BI-RADS) to report the results of the mammogram. This system recommends which follow-up tests may be needed after a mammography, as well as the risk that an abnormality may be cancer (malignant).

Breast Imaging Reporting and Data System (BI-RADS)



Assessment of mammogram

Recommended follow-up tests

Likelihood of malignancy (cancer)

Category 0: Incomplete

More mammogram images needed

Spot compression


Need to compare with an earlier mammogram


Category 1: Negative

Normal, no areas of concern

Continue with regular screening mammograms


Category 2: Benign

Normal, with non-cancerous areas based on their appearance

Continue with regular screening mammograms


Category 3:

Probably benign

An area that is probably non-cancerous

Follow-up mammogram in 6 months to watch for any changes to the area

Greater than 0% but less than 2%

Category 4A:

Low suspicion for malignancy

There is an abnormal area that is of low concern


Greater than 2% but less than or equal to 10%

Category 4B: Moderate suspicion for malignancy

There is an abnormal area of some concern


Greater than 10% but less than or equal to 50%

Category 4C: High suspicion for malignancy

There is an abnormal area of high concern


Greater than 50% but less than 95%

Category 5

Highly suggestive of malignancy

There is an abnormality that is very likely to be cancer


Greater than 95% chance

Breast density and your mammogram results

Your mammogram report may say you have dense breast tissue. Dense breasts have more connective tissue, glands and milk ducts than fatty tissue. Breast density is an inherited trait. Some studies show that women with dense breast tissue in 75% or more of their breasts are 4–6 times more likely to develop breast cancer than women with little or no dense breast tissue.

Breast density can only be seen on a mammogram, but dense breasts also make a mammogram harder to read. On a mammogram, fatty tissue looks dark, while dense tissue looks white, like tumours, so it can hide a tumour.

What happens if a change or abnormality is found

The doctor will decide if further tests are needed or not. Some tests that might be done include:

  • diagnostic mammography – if an abnormality was found on a screening mammography
  • breast ultrasound
  • biopsy
  • MRI

Digital mammography

Digital mammography uses an electronic image of the breast instead x-ray film. It is sometimes called full-field digital mammography.

Digital mammography is done the same way as standard film mammography, but the images are stored on a computer. The advantage of digital mammograms is that the images can be enlarged and changed so they can be looked at in more detail. They can provide better views of suspicious or abnormal areas. Images taken during digital mammography can be sent electronically from one location to another, and they are easily stored for future use or comparison.

Digital mammography may be used to further examine an abnormality found during a screening mammography or to diagnose breast cancer in centres where this equipment is available.

Mammography with breast implants

If you have breast implants, the technologist will make sure that as much breast tissue as possible is seen on the mammogram. Special techniques (called implant displacement techniques) are used to move the implant out of the way and pull the breast tissue forward so it can be in the image. Additional views may need to be taken to examine as much of the breast tissue as possible.

Extra care is taken when compressing the breasts to avoid rupturing the implant.

Studies show that women with breast implants are diagnosed with breast cancer at a similar stage and have a similar prognosis as women who do not have breast implants.

Mammography safety

Modern mammography equipment uses very low doses of radiation, but repeated exposure to x-rays does increase the risk of developing cancer. The benefits of mammography and finding breast cancer early outweigh the risk of repeated exposure to radiation.

Some women worry about radiation to the thyroid during mammography. The Canadian Association of Radiologists (CAR) does not recommend using a thyroid shield during mammography. Find out more about the CAR position statement on the use of thyroid shields.

Compressing the breast during mammography does not cause breast cancer. Compression doesn’t make tumours that are already there grow or spread any faster.

Limitations of mammography

Mammography is the best test we have to find breast cancer early, but it is not perfect.

A false-negative result means that the mammogram results are normal but the woman actually has cancer.

Mammography can also give a false-positive result. This means that the mammogram result suggests that a woman may have breast cancer when she really doesn’t. Further tests will show that many of these abnormalities are not cancer.

Expert review and references

Breast density

If you have had a mammogram and been told you have dense breasts, what exactly does this mean?