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Cancer prevention | Do you know your breast density?

Mammograms miss about 50% of breast cancers in the densest breasts

Person Analyzing a mammogram

In 2014, Michelle Di Tomaso was sidelined after being diagnosed with breast cancer. It came as a shock since her last four mammograms revealed no issues. But there she was, facing multiple surgeries, chemotherapy and radiation. She would go on to spend 25 days in hospital and undergo three unsuccessful reconstructive breast surgeries.

She later learned that her diagnosis had been delayed because her dense breast tissue had camouflaged the tumor. Had the cancer been caught earlier, she was told, her treatment would have likely been less invasive.

Today, Di Tomaso is working with other breast cancer survivors and health care professionals, to help other women avoid the same fate. Through Dense Breasts Canada, the women have three goals: (1) to increase awareness about the risks associated with dense breasts; (2) to make it mandatory to report breast density to family doctors and women; and (3) for health providers to give ultrasounds to women with extremely dense breasts and other high risk factors.

We sat down with Dr. Carl Bloom, Director of Radiology at Medcan, to help us understand breast density and his recommended prevention and treatment plans.

What is breast density and how common is it?

Breast density is comprised of everything that makes up the breast tissue other than fat. That includes: the supporting tissues made of collagen, as well as ducts, and glands. Fat appears black on a mammographic image. Collagen, ducts and glands appear white.

Approximately 60% of women under the age of 50 have dense breasts. That percentage decreases as women age to approximately 30% of women aged 50 and older. Paradoxically, despite breast cancer risk rising with age, breast tissue becomes fattier.  This paradox is likely explained by a combination of  unrelated factors (slowly growing malignancies manifesting later in life, faulty gene repair mechanisms associated with ageing, latency in developing malignancies related to prior insult to breast tissues (radiation, viruses, severe hormonal fluctuations, alcohol use, etc.).

Does breast density increase your risk of breast cancer?

The answer is yes. Women who have dense breasts – i.e. greater than 75% fibroglandular tissue elements in their breasts based on mammography stratification – have anywhere between two to four times the risk of breast cancer in their lifetime (2x greater than average, and 4x greater than a woman with completely fatty breasts). The latter risk (4x) supersedes the risk of breast cancer in women with a first-degree family history of breast cancer.

Furthermore, increased breast density can lead to masking of breast tumors on mammography, which reduces the sensitivity of the examination (50% sensitive in very dense breasts and 90% sensitive in fatty breasts).

Visual comparison of cancer in fatty breast vs dense breast

Image courtesy of Dense Breasts Canada

Across the board for all women, the risk of developing breast cancer in a lifetime is approximately 10%. If you are genetically predisposed, the risk could go as high as 50%.

What increases the risk of having high breast density?

Your genetics. It’s known that some women have high dense breasts all through their lives, while some women’s breasts get fattier with age. This is thought to be predetermined genetically.

Women who are on hormone replacement therapies.  We know that HRT causes increased breast density; and we do know that there is a purported higher risk of breast cancer in women who receive unopposed estrogen replacement. This has led to tailoring or stoppage of HRT in post-menopausal women.

The reverse is also true: if you are on Tamoxifen, a drugs who’s metabolites act as an estrogen-receptor antagonist, a reduction in breast density is noted mammographically  as well as a reduction in the  incidence of breast cancer.

Should women who have high breast density on a mammogram be told?


They should be told everything they need to know about their risk of breast cancer and be encouraged to continue regular testing-screening.

What else should women with dense breasts do to prevent missed diagnoses?

I’d suggest that instead of only having a mammogram every two years, they should have one annually, This practice is recommended by Radiological societies and most Oncologic societies in North America.

Should women consider alternative or adjunctive testing?

I believe that women should still have an annual mammogram, because that’s a critical screening test. It is a particularly sensitive screening test in women with fatty breasts and also can help stratify breast cancer risk by identifying high breast density.

But should they have MRIs instead? The answer is yes in a perfect world.

With MRI testing, the MRI identifies up to 14 breast cancers per 1,000 women screened. Mammograms pick up 4 cancers per 1,000 women screened with dense breasts and between 6-8 cancers in woman with fatty breasts.

There are, however, many challenges with MRI: availability, cost, and the low specificity of the modality (high false positive rates) which leads to more testing and biopsies.  It’s a huge cost to society, and ultimately very stressful to the patient. As well, studies indicate that patients are more likely to cancel their screening MRI studies due to claustrophobia, fear of injectable dyes etc). In patients who have a high risk of breast cancer, such as those having dense breasts on mammography AND a first degree relative with a diagnosis (mother, sister or daughter) or those patients who have a previous diagnosis of breast cancer; OR those patients with a genetic mutation (i.e. most commonly:  BRCA I and II) – the lifetime risk and pretest probability of cancer supports adjunctive MRI screening.

Should these women get breast ultrasound?

Dense Breasts Canada wants health providers to provide ultrasounds to women in Category D (Extremely Dense Breasts) and to women in Category C with a family history (Heterogeneously Dense).

A large study of breast ultrasound screening in dense breasts (the ACRIN 6666 trial) has concluded that ultrasound alone will identify four additional breast cancers per 1,000 patients than mammography alone. In other words, ultrasound screening would be doubling the chance of identifying a breast cancer when compared to mammography alone. Furthermore, the tumors identified on ultrasound tend to be small, receptor positive, locally invasive malignancies, with no nodal involvement. The prognosis of women diagnosed with breast cancer is heavily dependent on negative nodal status and positive receptor status.

Breast ultrasound adjunctive screening has been our practice at Medcan for the past 10 years long before this was adopted by the general imaging community.  We have long believed, based on personal experience as breast imagers and supported by data from large Imaging trials, that this was the best practice for our patients and referring doctors.

In summary, in lieu of having a breast cancer pick-up rate of 4/1,000 on mammography for women with dense breasts, with the addition of ultrasound we can double the rate of breast cancers diagnosis (rate of 8/1,000). MRI is considered the gold standard (nearly 100% sensitivity and can identify up to 14 cancers/1,000; so the assumption is that 6 cancers per 1,000 women screened go unnoticed in women with dense breasts despite screening with mammo and ultrasound. These tumors are invisible to mammography and ultrasound and are non-palpable.

How do you identify the other 6 cancers?

First ask: Do they need to be identified?  The answer is unequivocally yes. Large breast imaging trials have shown between a 20 and 40% decrease in breast cancer deaths through screening. Therefore, it is important to encourage breast screening for all women starting at 40 years of age and earlier if the woman’s risk profile suggests a high lifetime risk of breast cancer. High breast density is determined by mammography and therefore this risk factor is only known after the patient’s first screening test.

In the U.S., 36 states have made it mandatory  that you have to report breast density to the patient and eventually it will be all states.

How is breast cancer diagnosis changing?

There’s digital mammography, which has a much higher pick up than analogue (film) mammography. There’s also 3D tomosynthesis mammography, which is becoming adopted widely. It’s a tomographic mammogram that has been shown to reduce callback rates for women undergoing screening. The drawback is a slightly higher radiation risk to the patient – a problem that is being addressed by physicists and will likely be rectified in the near future.

What is your main message for women who want to be vigilant in their breast cancer prevention?

With the knowledge and certainty that breast cancer death rates are reduced through careful breast screening, it is absolutely necessary to encourage breast screening. The question is how frequent and with what modalities? In women diagnosed with dense breasts, the answer is every year – with an adjunctive ultrasound, or an MRI, if it is available. Unfortunately, in the Canadian system, you can’t easily access breast MRIs for every patient due to resource limitations. Just as importantly, breast MRI leads to 25% more testing on patients who are ultimately found to have no cancer. The cost to society and to patients is enormous. MRI should be used judiciously in patients who have a high pre-test probability of cancer. In that select group of patients, the chance of a false positive result is diminished.

The Annual Health Assessment at Medcan includes mammography and ultrasound. Book yours here. Learn more about the Dense Breasts Canada’s campaign here.

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