Dense breasts and what this means for women undergoing breast screening
Updates on breast screening in women with dense breast tissues 24/04/2026
Three different supplemental imaging techniques have been compared in a UK randomised clinical trial called the BRAID UK trial https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(25)00582-3/fulltext
The level 1 or highest quality evidence shows that when compared to standard mammograms combined with ultrasound:
- Contrast-enhanced mammogram (CEM)
Or - Abbreviated breast MRI
Are:
Equally effective in detecting new breast cancers
Detect 3 times as many invasive cancers
Detect earlier stage cancers half the size; and may influence / improve survival outcomes
Frequently asked questions:
1. Does having dense breasts increase my risk of breast cancer?
- Increased breast density (C or D) increases risks of developing breast cancer compared with predominantly fatty breasts
- Increased risk is moderate and requires consideration alongside other risk factors: family history, prior biopsies, and genetic predisposition
2. Should all women with dense breasts have supplemental screening?
- Supplemental imaging should be discussed as part of shared decision-making
3. What are the main supplemental imaging options?
- Contrast-enhanced breast MRI (Full MRI or “abbreviated/fast” MRI)
- Abbreviated MRI takes 15 minutes and is equally effective and less costly
- Contrast-enhanced mammography
- Automated breast ultrasound aided by a machine rather than hand-held is less effective compared to MRI or contrast mammogram
4. How do I know if I personally should have supplemental imaging?
- Shared-decisions are best guided by your 5-year risks of developing breast cancer described as High risk (greater than 1.66% and less than 6%) or Very High risk (greater than and equal to 6% at 5 years)
- You and your Consultant Breast Cancer Surgeon should weigh potential benefits (earlier detection) against harms (false positives which means “over detection” where a lesion that enhances is not a cancer; including potential increases in anxiety; and in costs to clients and providers)
What is a Contrast-Enhanced Mammogram or CEM?
This is a mammogram done after injecting an iodine-based contrast dye into a vein, so that the scan can highlight areas with increased blood flow, such as many cancers
It uses the same X-ray exam as a standard mammogram, but takes paired low energy and high energy images and then combines them to create an iodine-only image that shows enhancing areas more clearly.
CEM can help detect cancers that are harder to see on a standard mammogram, especially in dense breasts (C or D). It can also be used in circumstances to assess the extent or true size of a known cancer, to check symptoms or to assess the response to upfront medical treatments like chemotherapy or endocrine treatments like Letrozole.
Practical point:
It is not the same as a 3-D mammogram which is called Tomosynthesis. It is the contrast that adds the extra information.
How does it compare with breast MRI?
Neither is universally “better”
Diagnostic performance:
CEM has a higher specificity (77%) which means fewer recalls and false positive biopsies than MRI
CEM has a lower sensitivity and cancer detection of 61% versus 100% for MRI
SO there is a slightly better overall discrimination for MRI (higher diagnostic odds ratio)
MRI is still preferred for:
- High-risk screening (High risk genes like BRCA; TP53; PALB2)
- Detailed local staging like:
- Multifocal / multicentric cancers
- Contralateral occult cancer which means cancers not seen on mammogram, like invasive lobular cancers (ILC) or non-calcified DCIS or ductal cancer in situ which is defined as intraductal or microscopic pre-invasive breast cancer cells
- Implants and lesions close to chest wall / axillary tail (upper outer breast tissue that extends into the axilla / armpit)
Practical points and patient-centred factors:
- CEM is cheaper, quicker and easier to implement in mammography units where MRI may not be readily accessible
- Women report a preference for CEM as there is no tunnel, and they feel less claustrophobic
Safety and contraindications:
- CEM uses iodinated contrast plus iodinated radiation
- It is not advised where there is a significant iodine allergy; untreated overactive thyroid called hyperthyroidism; requires caution where there is renal / kidney impairment
- It is generally avoided for high-risk mutation carriers as a primary screening tool because of radiation
*End of update*
June 2022
General importance
The density of breast tissues relates to the amounts or relative proportions of normal breast duct tissues that appear as the white component on an X-Ray or mammogram. This breast duct tissue is referred to as fibroglandular that means breast ducts surrounded by fibrous or connective tissues (collagen, fibrinogen). Connective tissues give structure and support to the breast ducts. It is the fibroglandular tissues that are assessed for any abnormal changes on mammograms.
Fibroglandular tissues absorb X-Rays and project as “white” on the mammogram. This is referred to as “dense” tissue. The amounts of fibroglandular tissue are compared to the amounts of fatty tissue that projects as “dark” on the mammogram as fat cells are mostly liquid, and are not dense like fibroglandular tissues. The amounts of fibroglandular tissue are largely genetically determined, and also depend on hormonal stimulations such as the oral contraceptive pill, the intrauterine Mirena coil and hormone replacement treatment (HRT).
Most cancers absorb X-rays to a similar extent as fibroglandular tissues, and therefore they also appear as white “masses” on a mammogram. Dense “white” tissues can therefore hide similar dense “white” cancers. (It’s like looking for a white ball in a snowstorm). This means that mammographically dense breasts have a reduced sensitivity (reduced detection of breast cancers) when relying on a mammogram only to detect breast cancers. In these cases, it is crucial to recommend additional screening using bilateral breast ultrasounds or breast MRI (Magnetic Resonance Imaging).
Assessments
A mammogram is the international standard for breast screening by “taking a picture” of each breast as a whole. The mammogram measures the amounts of fibroglandular tissues. The principle of an X-ray is to identify the “white” from the “dark” areas to calculate the ratio of how much “white” to “dark” areas there are in both breasts.
It is not possible to assess overall breast density using breast ultrasounds. The latter is operator-dependent and doesn’t take a picture of the whole breast, and is used to focus on a particular lesion seen on the mammogram. Ultrasound can’t report overall fibroglandular density in the breasts.
Below is a picture that shows how a cancer would present on a mammogram in each of the breast density categories.
In a fatty breast (A or B) a small cancer is easily seen, compared to a dense breast (C or D), whereby a large cancer is difficult to see.
Definition and measurements
Mammographic breast tissue density is classified into 4 categories by the ACR American College of Radiology and BI-RADS Breast Imaging – Reporting and Data Systems as follows:
Breast tissue density is reported from A to D by the Consultant Radiologist as follows:
|
A |
The breasts are almost entirely fatty |
about 10% of the screening population |
|
B |
Scattered areas of fibroglandular density |
about 42% of the screening population |
|
C |
heterogeneously dense, that may obscure small masses |
about 40% of the screening population |
|
D |
extremely dense, that lowers the sensitivity of mammography |
about 8% of the screening population |
ACR BIRADS C and D are clinically regarded as dense breasts
Radiologists measure the breast density on mammograms using different methods. Most commonly the Radiologist “eye-balls” the films and visually quantitates proportionate areas of “white” compared to “dark” tissues. Alternative methods use different automated computer programs potentially reducing variabilities in the reporting by Radiologists.
Implications
Dense breasts are a strong independent risk factor in all women and are currently used for breast cancer risk prediction. Dense breasts (ACR-C and D) raise breast cancer risks two fold above the current risks of 1 in 7- 8 women developing breast
cancer. This rate equals that of having a first degree relative such as either a mother or sister having breast cancer. Dense breasts account for 26% of cancers in post-menopausal women.
Extremely dense breasts (ACR-D) raise breast cancer risks by 4 to 6 fold compared to extremely fatty breasts (ACR-A).
A personal breast cancer risk test called MammoRisk uses an Artificial Intelligence (AI) – developed algorithm to assess breast cancer risks at 5 years. Breast density is one of the strongest prognostic factors for increasing breast cancer risks that is integrated into the MammoRisk test.
Recommendations by the European Society of Breast Imaging (EUSOBI)
X-ray based imaging techniques are all significantly affected by dense breast tissues leading to an under-diagnosis of breast cancers. The diagnostic sensitivity (optimal breast cancer detections) of mammograms is highest in women with fatty breasts (ACR-A) at 86- 89%, compared to women with dense breasts where the detection of breast cancers falls to 62-68%.
Current European recommendations are to perform bilateral synchronous ultrasounds with the potential to increase the overall imaging sensitivity from 77% to 91% in women aged from 40-49 years. Overall, however, ultrasound may be limited compared to performing contrast enhanced breast MRI. Supplemental MRI in addition to mammograms detected an additional 16.5 cancers per 1000 screening episodes.
Breast MRI assesses increases in tissue blood flow where cancers that comprise faster growing cells “light up” significantly more than normal or benign tissues like cysts or fibroadenomas. Results of a Dutch clinical trial called the DENSE trial ultimately concluded that a potentially more cost effective screening strategy in women with dense breasts would be to recommend 2 yearly mammograms and MRI that saves 8.6 lives per 1000 screened women. This is an evolving field and “we should watch this space”.
Sharing information with women
All women with dense breasts should have the advantages and disadvantages of enhanced breast screening using combined mammograms and MRI 2 yearly explained to them.
Advantages of enhanced breast screening:
Two yearly MRI leads to early cancer detection in about 10% of women and reduces the risk of dying from breast cancer to a little over 3%, providing a mortality reduction by about 40 %. A woman gains on average 15 years in good health, before she dies of another cause.
Disadvantages of enhanced breast screening:
Enhanced screening with breast MRI in particular will increase the chance that she will at least once experience the situation of a ‘false alarm’, i.e. receive a positive screening test which, after appropriate assessment, turns out to be a harmless finding. Of all positive (abnormal) screening findings, only about 30% are really cancerous; this value is similar for mammography and for MRI.
All women should be made aware of the clinical implications and importance of each mammogram report classifying their individual breast tissue density score labelled from A to D. This is now a mandatory requirement to all screening mammogram reports.
Breast tissue density informs future screening recommendations such as bilateral breast ultrasounds and or breast MRIs at every screening round. This approach is preventative in women and will save lives.
Updated 25/09/2025
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