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Breast cancer: Why breast density matters

Breast cancer: Why breast density matters
Dense breasts increase both the risk of developing breast cancer and the likelihood that cancer will be missed on a mammogram
Despite improvements in breast screening and treatment, the World Health Organisation reported that 685,000 people died of breast cancer worldwide in 2020, and that 2.3 million women were diagnosed with breast cancer.
Mammography is the recognised protocol for breast cancer screening, but mammograms, which produce x-ray images of the breasts, can be less effective when examining women with dense breast tissue.   

What are dense breasts?

A mammogram scan showing the four types of breast tissue density
Dense breast tissue refers to the appearance of the breast tissue on the mammogram.
A woman's breast is composed of three types of tissue; the proportions vary with each woman. The more glandular and fibrous connective tissue a woman has in her breasts, the "denser" her breasts are.
Dense breasts are normal and are rated into one of four categories on a mammogram.
"The more glandular and fibrous connective tissue a woman has in her breasts, the 'denser' her breasts are"
Dense tissue shows as light grey or white on a mammogram. As breast density increases, the breast tissue becomes more and more of a white-out situation.
This matters because cancers also display as light grey or white on a mammogram and can be “hidden” in dense tissue.
The four types of breast density are:
  • Fatty—more common in older women, most of the breast is fatty breast tissue which shows up as primarily dark grey or black, making it easier to spot a tumour  or other irregularities.
  • Scattered areas of fibro glandular—this refers to a combination of primarily fatty tissue and some dense fibrous and glandular tissue.
  • Heterogeneously dense—large portions of the breast tissue are dense. It’s hard to see masses in these dense breasts, as they might appear as white shadows.
  • Extremely dense—most of the breast is made of dense breast tissue. This makes it very hard to find cancer.

How dense breasts can affect your mammogram

Clare Cowhig’s story shows how crucial it is for women to know about breast tissue type and cancer risk.
Aware of her strong family history of breast cancer, Clare did what she thought was the right thing—she started having mammograms at age 41.
She was always given the all-clear, except at age 49 when one area was examined by ultrasound after her routine mammogram. Again, it appeared that nothing sinister could be seen on the scans.
What Clare did not know was that she had dense breasts.
Clare says, “On showering one day, I noticed an unusual area of thickening on my right breast. I decided to book a whole breast private ultrasound to put my mind at rest."
Imagine her surprise when, at age 50 and only nine months after her last examination, the sonographer she saw privately announced, “You have the densest breast tissue I have ever seen!”
"If I'd been told I had dense breasts, I would have sought additional screening"
The sonographer identified a highly suspicious area on the right breast and another, which also required further investigation, on the left. Additional scans, breast MRI and multiple biopsies at the hospital confirmed that Clare had invasive ductal, bilateral breast cancer.
Clare explains: "I had a tumour in each breast. The one on the right was stage-three and over five centimetres: the one on the left was stage two and two centimetres. I had areas of in situ disease in both.
"I now know these tumours never appeared on my mammograms because of my highly dense breast tissue."
Clare needed multiple surgeries, including a double mastectomy and chemotherapy, to remove the cancers in her breasts.
She says, “If I'd been told I had dense breasts, I would have sought additional screening. I would have been even more vigilant. My tumours could have been found at a smaller and less advanced stage, and I wouldn't have had to endure such extensive treatment."

Finding breast cancer early is key

Breast cancer is far easier to detect in a fatty breast than in a dense breast
French resident Cheryl Cruwys’ experience with cancer is somewhat different.
Although her initial mammogram at age 50 was clear, the images showed some dense tissue on the left breast. To complete the assessment, the radiologist performed an ultrasound.
Cheryl says, “The ultrasound scan revealed an area of concern that was not seen on mammography, and the subsequent biopsy confirmed a small invasive cancerous tumour measuring eight millimetres."
Understandably, Cheryl appreciated her thorough examination. Having both the mammograms and ultrasound tests done simultaneously took her by surprise.
She explains, "The French breast screening programme offered ultrasonography, an additional approach for dense breast tissue. This is what contributed to my early diagnosis.
I had a lumpectomy and six weeks of radiotherapy. Thankfully, I did not need a mastectomy, reconstructive surgery, or chemotherapy."
"A mammogram alone on dense breasts is insufficient"
Motivated by her new knowledge of breast density, the retired UK teacher initiated Breast Density Matters UK, a patient advocacy organisation aiming to raise awareness and educate women about dense breasts.
To learn more about screening and risk implications of dense breast tissue from medical experts, she suggests visiting the world’s leading resource on the topic.
Cheryl's take-home message is clear. “A mammogram alone on dense breasts is insufficient. A normal mammogram does not always indicate the absence of cancer. When getting a mammogram, women should try to understand dense breasts and inquire about their breast density.”
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