Mammography or breast ultrasound: What’s the difference?

Mammography (or a mammogram) is an X-ray of the breasts using low doses of radiation. It produces two-dimensional images that reveal certain internal structures. This exam can detect small anomalies such as cysts, calcifications and small tumours, which cannot be detected by palpation. The test includes two X-rays of each breast: one horizontal and one sideways. To obtain optimal image quality, compression is applied to the breast for a few seconds at a time.

Since 1998, Quebec’s ministère de la Santé et des Services sociaux (MSSS) has been offering a free, voluntary screening program that invites women aged 50 to 74 to have a mammogram every two years.

To have access to the program, women aged 50 or older must receive an invitation letter, which is mailed to them every two years. This letter serves as a medical prescription and is accepted at all centres designated by the MSSS [2]. A list of these centres and appointment lead times can be consulted online [3]. It is also possible to take part in the program by obtaining a prescription from a health care professional.

A screening mammogram cannot detect all cancers, especially the smallest ones. Therefore, it is preferable to repeat the test at regular intervals. A false-negative result may reassure women, leading them to overlook other symptoms or abandon the screening program.

Mammography can also detect lesions that are not cancerous (false positives), leading to more intrusive procedures (biopsies). False-positive results are more common in younger women. Over a 10-year period, half of all women will have a false-positive result, most often on their first mammogram. This probability decreases considerably when the radiologist has a previous mammogram for comparison. Naturally, false positives often cause a great deal of anxiety [4].

A more sensitive version of the examination is called “tomosynthesis” or “3D mammography.” The precision of images in three dimensions reduces the need for additional tests or follow-up appointments. In addition, it improves breast cancer detection up to 30%, especially for small cancers that can be hidden by overlapping tissue in 2D mammography [5].

Breast ultrasound is a complementary examination to mammography. Painless and effective, it can determine the nature of the lesions observed in order to clarify the diagnosis (cysts, etc.), which a mammogram cannot do.

After applying a contact gel to the skin, a probe emitting high-frequency sound waves is moved over all surfaces of the breast. These waves are translated into visible images on a screen.

Ultrasound is mainly used for the following purposes [6]:

• To establish the nature (liquid or solid) of masses felt by palpation or of an image deemed abnormal (or uncertain) during mammography
• To replace mammography when it is contraindicated in case of pregnancy
• To visually guide the physician when performing a breast biopsy

It is important to point out that a breast ultrasound is not a breast cancer screening test and will never replace a mammogram [7].

Thanks to the development of diagnostic techniques and new treatments, the survival rate after a breast cancer diagnosis is over 80% after five years. The mortality rate from this type of cancer has been halved over the last 40 years [8]. Half of this reduction is believed to be due to widespread mammography screening programs [2].