Why Supplemental Imaging Matters in Manitoba 

Screening for Dense Breasts

Equity for women with dense breasts

Manitoba must offer supplemental screening for women with Category C & D dense breasts.

Women with dense breasts face discrimination since mammograms are their only option for breast cancer screening. They deserve equal access to early detection, just like women with non-dense breasts. Early detection saves lives and reduces the need for chemotherapy and mastectomy.

Breast density is assigned based on the appearance of breast tissue on a mammogram. Radiologists categorize breast density using a standardized system called the Breast Imaging Reporting and Data System (BI-RADS), which divides breast density into four categories: A, B,C, or D. Category C and D are considered to be dense breasts.

Here are some tips to have a conversation with your doctor to request an ultrasound.

Manitoba Briefing Note - June 2025

The radiologist assigns a density category after evaluating the mammogram, and this information is included in the mammogram report. Women with heterogeneously dense or extremely dense breasts are considered to have high breast density.

By offering supplemental screening to women with dense breasts, many more cancers could be detected early, potentially saving lives. Dense breast tissue makes it harder to detect cancer on mammograms and also increases the risk of developing breast cancer. As a result, women with dense breasts are often diagnosed with larger, more aggressive cancers, leading to a poorer prognosis.

Benefits of ultrasound for women with dense breasts

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 mammograms.

Several large studies have shown that supplementing mammograms with ultrasound can increase detection.

It has been known since 1995 with a Canadian study that ultrasound finds significant numbers of additional cancers missed on mammograms. That study spurred more research, all showing the benefit of ultrasound. In 2022, a study published by Vancouver authors showed they found 7 additional cancers per thousand women using supplemental screening ultrasound exams. All cancers were small and node-negative. Significantly, 40% were in women with no family history and 60% were in women with Category C density.

Preliminary findings from Japan’s J-START RCT show a reduction in interval cancers by half. It would be unethical to wait for J-START to show mortality reduction since reduced interval cancers precede mortality reduction.

This comprehensive article on dense breasts and supplemental screening outlines the risks of dense breast tissue, and the current and future modalities that can detect cancers missed on mammograms.

Annual screening with mammography leads to fewer interval cancer cases: A Canadian study showed that annual screening with mammography for women with Category D density leads to fewer interval cancer cases.

Benefits of MRI for women with Dense Breasts

MRI offers significant benefits in detecting cancers in people with dense breasts. It provides a clearer and more detailed image compared to mammograms, making it easier to identify tumors that might be obscured by dense breast tissue. MRI is highly sensitive, capable of detecting small and early-stage cancers that mammograms might miss. This enhanced detection can lead to earlier diagnosis, allowing for less aggressive treatment options and potentially better outcomes. 

Overall, MRI is a valuable tool for improving breast cancer detection and prognosis in individuals with dense breast tissue. Based on new RCT data from the Netherlands, the European Society of Breast Imaging now recommends MRI every 2 to 4 years for all women in category D. Yet, in Manitoba, screening ultrasound is not easily accessible for women with dense breasts.