What Is Breast Density?
Each woman’s breast composition is unique. Every woman has fat, glands and fibrous tissue in her breasts, but the proportions of each vary woman to woman. There are 4 categories of breast density.
Each woman’s breast composition is unique. Every woman has fat, glands and fibrous tissue in her breasts, but the proportions vary woman to woman.
Breasts that have more than 50% glands and fibrous tissue are called dense breasts.
Dense breasts are common. For women aged 40-74 years, 43% have dense breasts. Breasts usually become less dense and more fatty with age:
- 56% of women in their 40s have dense breasts.
- 37% of women in their 50s have dense breasts.
- 27% of women in their 60s have dense breasts.
It is normal to have dense breasts, but women need to know if they have dense breasts so that they can understand the implications.
Your breast density is determined by the radiologist reviewing your mammogram. Automated software can also calculate the breast density, but is only used in Nova Scotia, to date. Breast density is NOT determined by the size or firmness of your breasts. You cannot tell density by look or feel. Some health practitioners after a clinical exam will tell women they have dense breasts, but it is not possible to know from feel.
Dense breasts are not the same as lumpy breasts.
Breast density is described using 4 categories, but the names of categories vary by province.
Some provinces divide the 4 categories into percentages (0-100%). Some provinces use letters (A, B, C, D). Some provinces describe the categories in words.
** ABOVE 50% / CATEGORY C and D ARE CONSIDERED DENSE BREASTS.
Dense breast tissue may obscure small tumors and make a mammogram less accurate.
UNDER 25% dense tissue | Category A | Almost entirely fatty
The breasts have little fibrous and glandular tissue and are mostly fat. A mammogram would likely detect an abnormality.
25%-50% dense tissue | Category B | Scattered areas of fibroglandular density
The breasts have scattered areas of fibrous and glandular tissue, but also quite a bit of fat.
51-74% dense tissue | Category C | Heterogeneously dense
The breasts have consistently distributed areas of fibrous and glandular tissue, making it hard for small masses to be detected by mammogram.
75% and above dense tissue | Category D | Extremely dense breasts
The breasts have a lot of fibrous and glandular tissue, making it even harder for a cancerous tumor to be detected by mammogram.
Which category are you in?
- 10% of women have the lowest category of density known as ‘Fatty’ breasts
- 40% have ‘Scattered areas’ of density
- 40% of women fall in the category Heterogeneously Dense Breasts/Category C/50-75% (considered dense breasts).
- 10% of women fall in the highest category ‘Extremely Dense/ Category D/>75% (considered dense breasts).
The higher the density, the higher the risk of breast cancer and that a tumour will be missed.
AGE: Dense breasts are more common in younger women. As women get older, their breasts usually become less dense, but not always.
- 56% of women in their 40s have dense breasts.
- 37% of women in their 50s have dense breasts.
- 27% of women in their 60s have dense breasts.
MENOPAUSE: Density usually decreases after menopause, but not always.
ETHNICITY: A greater percentage of Asian women have dense breasts.
HEREDITY: Breast density can be inherited.
HRT: Density increases in women on HRT.
BREAST SIZE: Smaller breast size is related to higher density.
PREGNANCY/ BREASTFEEDING: Breasts become denser during this time.
WEIGHT: A higher BMI is inversely related to density.
AROMATASE INHIBITORS AND TAMOXIFEN: The use of these cancer drugs decreases density.
Studies show that regular mammograms reduce mortality. Even if you do not have dense breasts, other factors may still place you at increased risk. Talk to your doctor/nurse practitioner and discuss your history. Women considered to be at high risk will likely be sent for an MRI, as well as a mammogram. Screening saves lives.
Why Breast Density Matters?
Dense breasts make it harder for doctors to spot cancer because both cancer and dense breast tissue appear white on mammograms. A camouflage effect is created.
Dense breasts make it harder for doctors to spot cancer because BOTH CANCER AND DENSE TISSUE APPEAR WHITE on mammograms. A camouflage effect is created. Fatty tissue allows greater x-ray penetration and shows as black or dark grey on a mammogram; the contrast makes cancer easier to detect in fatty breasts. Mammograms are 98% accurate in women with fatty breasts, but only 50% accurate in women with the densest breasts (Extremely Dense Breasts/ Category D, >75% dense tissue).
Kolb, T., Lichy, J., & Newhouse, J. (2002). Comparison of the performance of screening mammography, physical examination, and breast US and evaluation of factors that influence them: an analysis of 27,825 patient evaluations. Radiology. (225):165–17.
Is 3D Mammogram (TOMOSYNTHESIS) more effective for dense breasts?
Tomosynthesis (3D mammogram) is available in a few provinces as a screening tool, but for now is mostly used as a diagnostic tool. It increases the detection of invasive cancer by an additional 1-2 cancers per 1000 women. However, its effectiveness declines as the density increases. Tomosynthesis does not detect as many cancers in women with the densest breasts (women with over 75% dense tissue/ Extremely Dense Breasts).
Supplementary ultrasound detects more cancers in dense breasts than Tomosynthesis. The evidence for the past 40 years shows that mammography is not enough for women with dense breasts.
The two greatest risk factors for breast cancer are being a female and getting older. Breast density is one of many risk factors.
A recent large scale study shows breast density is a more prevalent risk factor than family history. The greater the density, the higher the risk of getting breast cancer.
Women with the highest level of density (Category D: >75% dense tissue) are 4-6 times more likely to get cancer than women with the lowest (Category A: <25 % dense tissue).
As well, since mammograms detect about 50% of cancers in dense breasts vs. 98% in fatty breasts, cancer found in dense breasts is usually detected at a later stage, possibly making prognosis worse.
Boyd, N., Guo, H., Martin, L., Sun, L., Stone, J., Fishel, E., Jong, R., Hislop, G., Chiarelli, A., Minkin, S., and Yaffe, M. (2007, Jan.). Mammographic density and the risk and detection of breast cancer. New England Journal of Medicine, 356:227-236. View Study
Engmann N, Golmakani M, Miglioretti D, Sprague B, Kerlikowske K, for the Breast Cancer Surveillance Consortium. (2017, Feb 2.)Population-Attributable Risk Proportion of Clinical Risk Factors for Breast Cancer. JAMA Oncol. View Study
Finding Out My Density
If you have had a mammogram, here’s how you can find out your breast density in your province.
Breast density notification was implemented on Oct.15, 2018. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts. If your mammogram was prior to Oct.15, 2018: You can find out your density by using this “Access to Information” form
Under “Requested Info" write: “I want to know my breast density category."
News: Screening ultrasound is now covered under MSP for women with dense breasts, if the healthcare provider provides you with a requisition. As of November 2020, screening ultrasound is available at:
Vancouver: X-ray 505
Vernon: Category D only
Kamloops: Royal Inland Hospital
Shuswap Lake: if mammogram is done at Vernon
We will continue to update this list.
Breast density notification was implemented on Oct.1, 2020. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts.
Your healthcare provider also receives a report that has your density category. Please discuss your optimal screening. Ultrasound is recommended for women with dense breasts.
Currently, there are clinics in Alberta, such as Mayfair, Canada Diagnostics, EFW, Pureform Radiology and MIC Medical Imaging that offer screening ultrasound to women with dense breasts, paid for by Alberta Health Services. It is your choice where to get screened.
Breast density notification was implemented for women with >75% density in Nov. 2018. You will be informed of your density category in your mammogram results letter ONLY if your density is over 75%. You will be asked to return annually for a mammogram. Your healthcare provider is also told if your density is over 75%. However, dense breasts refer to breasts with 50% and over dense tissue. Women in the 50-75% category are currently not told they have dense breasts because this information is not recorded. However, the screening program manager has informed us that they are updating their system to be able to notify all women of their breast density. Due to the pandemic, there is no exact timeline right now.
Breast density notification was implemented on Jan.5, 2021. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts.
Your healthcare provider also receives a report that has your density category. Please discuss your overall risk and optimal screening. Evidence shows ultrasound is recommended for women with dense breasts as it finds additional cancers that have been missed on mammogram. When speaking with your healthcare practitioner, please consider taking this position statement from the experts at the Canadian Society of Breast Imaging with you.
You will be informed by mail of your density only if your density is over 75%. You will be asked to return annually for a mammogram. Your healthcare provider will also be told if your density is over 75%.
However, starting this spring, your provider will be informed of your specific breast density category (A,B, C, D). Please ask your provider what it says in the mammogram results letter.
Your mammogram report sent to your healthcare provider has a description of your density in percentages. Every report has the density. You will NOT be notified directly but you can find out your density by asking. The density information is on the third line of the report. Women with over 75% density and a family history are offered screening ultrasound. We continue to advocate for direct notification and if you would like to express your concerns about the withholding of health information, please contact The Health Minister, The Honourable Christian Dubé
Breast density notification for all women was implemented in Nova Scotia October 29, 2019. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts. If your mammogram was prior to October 29, 2019, contact NBSP at 902 473 3960 or 1 800 565 0548 for information on how you can request your breast density. Screening ultrasound is refused for women in Nova Scotia. If you would like to advocate, please contact Premier McNeil at email@example.com
Breast density notification for all women was implemented in New Brunswick in July 2020. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts. If your mammogram was before July 2020, please use the information form below to request your breast density.
Breast density notification for all women was implemented in January 2020. You will be informed of your breast density category in your mammogram results letter: A, B, C or D. Category C and D are dense breasts. The government committed to screening ultrasound for women in Category D.
You are not provided with any information. Your healthcare provider is notified if the breast density is over 75%. Women with over 75% density are asked to return for an annual mammogram. However, dense breasts refer to breasts with 50% and over dense tissue. Women in the 50-75% category are not told they have dense breasts because this information is not recorded. We have been informed that in early 2021 all women in Newfoundland will be informed of their breast density. We will keep you updated.
NWT and YUKON
You are not provided with any breast density information. Your healthcare provider is notified if the breast density is 75% and over. Women with dense tissue in the 50-75% category are not told they have dense breasts because this information is not recorded. In NWT, women with over 75% density are asked to return for annual mammograms.
What If I Have Dense Breasts?
Discuss your density category and any other risk factors such as genetics, family history, and previous biopsies so that your overall cancer risk is understood.
Discuss your density category and any other risk factors such as genetics, family history, previous biopsies, etc. so that your overall cancer risk is understood. Evidence shows dense breasts are a more prevalent risk factor for breast cancer than family history.
If you have dense breasts, talk to your doctor/nurse practitioner about having additional screening, such as ultrasound because a "NORMAL" mammogram result may not be accurate. Dense tissue can mask cancer. Ultrasound finds additional cancers. You are the best advocate for your health and you may need to advocate for additional testing.
HOW TO ADVOCATE FOR ADDITIONAL SCREENING
It is NOT Canadian protocol to order a screening ultrasound for women with dense breasts unless an abnormality has been detected by mammogram or clinical examination. Yet, we know early detection is key.
If you want added screening, advocate for yourself and let your doctor/nurse practitioner know you are aware of the following risks:
- The increased cancer risk associated with dense breasts. Dense breasts are an independent risk factor. Dense breasts are a more prevalent risk factor than family history.
- The unreliability of mammography for dense breasts because of the camouflaging effect. Both cancer and dense tissue appear white on a mammogram. This affects both categories of dense breasts.
- The high level of interval cancers for women with dense breasts- cancers detected in between screenings when a woman feels a lump after a negative mammogram.
- The proven ability of ultrasound and MRI to detect additional cancerous tumours missed by mammogram
Remember: In most cases, when extra screening is done, women are NOT found to have breast cancer.
- Speak with your doctor about your breast density, all breast cancer risk factors and your best screening options.
- Perform regular self-exams between screenings. A normal mammogram result may not be accurate.
- To improve early detection, consider additional screening, such as ultrasound (or MRI if you are considered high risk).
- Consider modifying lifestyle factors such as diet, exercise, alcohol intake and hormone use to decrease cancer risk.
- Continue to have mammograms because they can detect calcifications, which can be the earliest sign of cancer.
Ultrasound can detect a small cancer in an early stage before it spreads to lymph nodes.
In women with dense breasts, studies show that for every 1000 women screened, ultrasound finds another 2-3 cancers that were not seen on mammogram.
Several large studies have shown that supplementing mammograms with ultrasound can increase detection up to 97% in women with dense breasts.
It has been known since 1995, when the journal Cancer published the first paper, that ultrasound detects small, invasive node negative cancers in women with dense breasts.
An RCT of supplementary ultrasound screening is underway in Japan, and is showing greater cancer detection, and reduced interval cancers.
Ultrasound is used when an abnormality is detected by mammogram or clinical exam. It is not used routinely for screening because it is a sensitive test and can sometimes result in false positives – an abnormality that is not cancer. False positives are inevitable in screening and very few tests will show cancer. If you want to have an ultrasound, ask and advocate for yourself. Ultrasounds, in addition to mammography, are important for women with dense breasts because mammograms can miss cancers due to the masking effect of dense tissue.
If you have dense breasts and would like an ultrasound, but your doctor/nurse practitioner will not order it, you can look into private facilities in your province that offer ultrasound without a doctor’s requisition.
Ontario Residents: There are three private clinics offering 3D Automated Breast Ultrasound (ABUS). Private insurance may cover some of the cost. Clinics are located in Toronto and Barrie. ABUS was approved by Health Canada in 2011 as a screening tool. You do not need a doctor’s/nurse practitioner’s referral. More information about private options can be found in the Screening Options Section.
BC Residents: UPDATE January 2019. Screening ultrasound for women with dense breasts is now covered by MSP if your doctor/nurse practitioner is prepared to give you an ultrasound. If not, there are imaging facilities that provide hand held ultrasound without a requisition. If you do not have a requisition, screening ultrasound is an out of pocket expense in BC (around $250).
3D ultrasound, also know as Automated Breast Ultrasound (ABUS) uses sound waves. Thousands of images are created and software reconstructs images in 3D.
Automated Breast Ultrasound (ABUS) was approved by Health Canada as a screening tool and as an adjunct to mammography for women with no symptoms for breast cancer. Evidence shows it finds an additional 3.6 cancers per 1000 women screened. It is currently available privately in Ontario (Toronto and Barrie) and under provincial insurance in Alberta.
3D Automated Breast Ultrasound (ABUS) can increase the rate of breast cancer detection by 35.7% in women with dense breast tissue.
Some private insurance plans pay a portion towards the use of ABUS. Please check with your provider. More information about where you can access ABUS can be found below.
There are three private clinics offering 3D Automated Breast Ultrasound (ABUS) in Ontario. Private insurance may cover some of the cost. Clinics are located in Toronto and Barrie. ABUS was approved by Health Canada in 2011 as a screening tool. You do not need a doctor’s/nurse practitioner’s referral.
YES! Mammograms are proven to reduce breast cancer deaths. Mammograms can reliably detect suspicious calcifications. Ultrasound/ABUS do not replace mammograms; they should be used in addition to mammography.
A breast cancer risk calculator is a tool that can help you and your doctor better understand your risk and make informed decisions. We’re posting the TYRER-CUZICK 8 model because it’s just been updated to include breast density as a risk factor. It’s a well-studied and widely-used model that is easy and quick. Just click Imperial Units and if you know your density click BIRADS. If you don’t know your density, you can still use it and retry it when you find out your density. The model calculates 10 year risk and lifetime risk. Check it out.
Screening Methods for Canadian Women with Dense Breasts
The following screening tests are described below: Mammography, Ultrasound, 3D Ultrasound (ABUS), MRI, Tomosynthesis.
Mammography reduces mortality from breast cancer, but it is an imperfect screening tool for women with dense breasts because it misses cancerous tumors in dense breasts.
Mammograms can miss cancer about 50% of the time in the densest breasts. Therefore, women with dense breasts might want to consider additional screening with their mammogram. Ultrasound or MRI, when combined with mammography, significantly improves the rate of cancer detection in women with dense breasts.
In Canada, additional screening is not easily accessed. You may need to advocate for yourself.
What it is: Mammography x-rays the breast from different angles. A mammogram is the only way to find out breast density.
Positives: Over the past 30 years, mammography has helped reduce deaths through early detection by more than 35%.
Limitations: For women with dense breasts, dense tissue and cancer appear white on a mammogram, making it hard for cancer to be seen. Mammography uses low-dose radiation.
Accessibility: Screening mammography is available across Canada, except Nunavut. Digital mammography is widely available and is more accurate than analog mammography in dense breasts.
What it is: Ultrasound uses high frequency sound waves to image the breast.
Positives: When used with mammography, ultrasound increases the rate of cancer detection up to 97%. It detects an additional 2-3 cancers per 1000 women. It can detect cancer in dense breasts at an early stage. It is quick, safe, and non- invasive. No radiation is involved.
Limitations: Ultrasound is a very sensitive test and is generally used for diagnostic purposes, not screening. It may identify abnormalities that require additional investigation. Ultrasound alone is not recommended as a breast cancer screening tool.
Accessibility: A requisition for ultrasound is not usually given unless there has been a suspicious finding on a mammogram, MRI or a clinical breast exam. There may be a private clinic in your city where you do not need a doctor’s/nurse practitioner’s requisition and can pay directly. UPDATE: IN BC: Screening ultrasound is now covered by MSP for women with dense breasts if the family doctor/nurse practitioner is prepared to give the woman a requisition.
What it is: This is a 3D ultrasound. Sound waves and software reconstruct breast images in a 3D plane.
Positives: ABUS can find small, invasive, node-negative cancers missed by mammography. ABUS was found to detect an additional 3.6 cancers per 1,000 women screened after mammography.When used with mammography, its effectiveness is from 94 to 97%. It is radiation free.
Limitations: ABUS may identify abnormalities that are not seen on a mammogram. Some of these abnormalities may require additional ultrasound or biopsy. Remember to resist the extra anxiety of false positives because most abnormalities are not cancerous.
Accessibility: Currently, ABUS can be found in Ontario and Alberta
In Ontario, patients must pay out of pocket for ABUS, but some private/group insurance plans may cover part of the cost. ABUS can be found in 2 locations in downtown Toronto and also in Barrie.
In Alberta, ABUS is covered under the provincial health insurance plan (AHIC) for women with dense breasts who qualify. A number of clinics offer ABUS.
What it is: It uses magnetic fields, radio frequency pulses, and a computer to produce detailed images of the breast..
Positives: MRI detects the most breast cancers of any imaging. It is an effective screening tool for women with dense breasts and detects 18 or more additional cancers per 1000 women. No radiation is involved. MRI is generally recommended for women with the highest cancer risk (those who carry the breast cancer gene, women who have had chest wall radiation for treatment of lymphoma, and some with rare genetic diseases)
Limitations: MRI is likely to generate more false positives requiring biopsy. It requires intravenous contrast injection called Gadolinium. The FDA has released statements on the use of Gadolinium.
Accessibility: MRI is available across Canada, but it is not used as a screening tool. It is currently used along with mammography for high-risk patients (BRCA gene, strong family history, or history of radiation to chest). There are a number of private pay MRI locations in Canada, but a physician’s referral is required.
What it is: Also known as 3D mammography, it creates multiple thin slices of images so that overlapping tissue is less likely to hide a cancerous tumour. The patient is positioned exactly the same way as for standard mammography.
Positives: Tomosynthesis increases the detection of invasive tumours over digital mammogram by an additional 1-2 cancers per 1000 women. When used with mammography, it decreases the false positive rate.
Limitations: Tomosynthesis does not detect as many cancers in dense breasts as ultrasound. The radiation dose may be higher than regular mammography because image acquisition takes longer. When used along with regular mammography, the radiation dose is slightly more than double.
Accessibility: Currently, Tomosynthesis is mostly being used for diagnostic purposes after an abnormality is found. However, In Alberta, patients visiting certain clinics are screened with both 2D mammography and tomosynthesis. Clinical studies are taking place in Canada to evaluate the role of tomosynthesis in screening for breast cancer.
Why the Canadian Task Force Breast Screening Guidelines are dangerous
The guidelines were made by a panel of 14 members selected by the Public Health Agency of Canada and the College of Family Physicians of Canada.
The panel did not include any experts in breast cancer screening. The panel included a psychologist, an occupational therapist and a nephrologist (kidney specialist), family doctors, nurses, a chiropractor, and an emergency room doctor. The guidelines ignored the input of experts in breast cancer diagnosis and treatment.
The Task Force stated that including an expert in breast cancer diagnosis would bias the guideline development. The TF implied that because experts earn a living by diagnosing breast cancer, they place financial gain above the welfare of patients. We strongly disagree. In Canada, our radiologists have wait lists and their incomes do not benefit by increased demand.
The guidelines are used by 36 000 Canadian family doctors. The guidelines pertain to screening for 9 million women, aged 40-74. Many provincial health agencies have adopted the guidelines in their policies. The guidelines are scheduled to be revisited in ~6 years.
Thousands of experts think the guidelines should be rejected. The 2000 member Canadian Association of Radiologists issued a statement rejecting the guidelines.
The Canadian Society of Breast Imaging issued a statement rejecting the guidelines.
Here are Op-Eds written by breast cancer screening experts:
The guidelines are not based on current science. The studies used to make the guidelines are 30-50 years old and the technology used is obsolete. One of the major studies used was flawed. The guidelines underestimate the benefits of mammography, overstate the harms and ignore the risks of dense breasts.
The Task Force has ignored the evidence and does not recommend screening for women in their 40s.
Here’s why women in their 40s need mammograms:
- The incidence of breast cancer increases at age 40
- 16% of breast cancers are found in women in their 40s
- Half of fatal cancers are diagnosed by age 49
- 40% of the years of life lost to breast cancer are in women diagnosed in their forties
- Women who start screening at age 40 are 44% less likely to die from breast cancer than women who do not have mammograms
The evidence supports women in their 40s being screened. If they are not, cancers will be found larger and more are likely to die.
Based on 2 models, Dr. Martin Yaffe and Dr. Nicole Mittman estimated 400 women a year will die annually in Canada if the Task Force recommendation not to screen is followed.
A Canadian study of 2.8 million women screened for breast cancer for over 20 years showed that women 50 to 70 had a 40 % lower rate of dying from breast cancer than women who did not have mammograms. It also showed women aged 40 to 49 had a 44 % lower rate of dying from breast cancer when they participated in screening [mammography].
A recent study from Sweden showed 60% fewer deaths in the first 20 years after a cancer diagnosis, and 47% fewer deaths in the 20 years after a cancer diagnosis, in women who had mammograms, than in women who did not have mammograms.
What are the harms the Task Force asks women to consider?
A) Anxiety from a “false positive?”
The term “false positive” is misused by the task force. One might think that it means that a woman was told that she has cancer, when she does not. They use it when the mammogram shows something, and a woman is asked to return for further tests to see if cancer is present, or not. The vast majority of those women do not have cancer. Depending on the outcome of the additional imaging, sometimes a needle biopsy, done with local freezing, is performed. These are usually minimally uncomfortable.
Without doubt, being called back induces some stress, but that stress has been shown to be short-lived and is resolved when the negative results of the imaging are obtained. Usually when provided with the facts, most women are willing to put up with the stress in exchange for avoiding missing a cancer that could shorten her life or cause her to have to undergo more aggressive therapy.
Overdiagnosis is the theoretical possibility that some cancers found by screening would remain harmlessly hidden in the body until the woman died of some other cause. It is likely that this occurs to some extent, but it can only be estimated. The Task Force overstates the incidence of overdiagnosis. Most experts believe that it is in the range of 1% and 10%. Overdiagnosis in more likely in older women, who may have other illnesses (heart disease) and even other cancers, that they could die of, before breast cancer might. But the probability of overdiagnosis is vanishingly small in younger women.
The decision whether to participate in screening is up to individual women with advice from their health care provider. For there to be true shared decision-making, both a woman and her physician need to have clear and accurate information on the benefits and harms of screening.
Write your MP
Write Health Minister Patty Hajdu Patty.Hajdu@parl.gc.ca
Watch Health Critic Don Davies question the Health Minister
Read Our Brochure
The Dense Breasts Canada website is intended for the general purpose of:
1. Raising awareness about dense breasts and
2. Encouraging advocacy for the reporting of dense breast tissue to women.
We have included references, where applicable to ensure that the information provided is evidence based. However, we cannot guarantee that the information on the site is error-free or complete. The website is not intended to be a substitute for medical advice from a doctor. For answers to specific health-related questions, please consult your doctor. While we speak of Automated Breast Ultrasound (ABUS), we are not affiliated in any financial way with any private clinics that offer ABUS.