Dense breast tissue can mask detection of cancer on mammography. Tomosynthesis does not solve this problem (reviewed in [1]). While calcifications can still be seen, noncalcified masses can be hidden and only about half of invasive cancers have associated calcifications. The denser the tissue, the more likely cancer will be missed: 40% of cancers are missed on mammography in extremely dense breasts, as are 30% in heterogeneously dense breasts [2]. In the United States, laws in 38 states and the District of Columbia mandate some form of density notification in the mammogram results letter provided to women. Insurance laws have followed in many states to require coverage for screening MRI or ultrasound in women with dense breasts. The intent of density notification laws is to allow women to make informed choices about additional screening beyond mammography if they have dense breasts.

In most European countries, screening is organized and occurs by invitation from a national program. Until now, there has not been any invitation to additional screening beyond mammography for women with dense breasts, though there is often some opportunistic screening with MRI and even ultrasound in select patient populations (see https://densebreastinfo.org/europe/european-screening-guidelines/mapscreeningguidelines/, accessed 1 April 2022). The EUSOBI guideline newly recommends screening MRI every “two to four years” for women age 50–70 with extremely dense breasts, though benefits of at least biennial screening MRI from ages 50–75 have been shown [3].

While mammography has been proven in randomized controlled trials to reduce deaths from breast cancer, there is far less proof of benefit in women with dense breasts. In the Netherlands, Van der Waal et al [4] found a 41% reduction in breast cancer mortality from mammographic screening in women with fatty breasts (relative risk, RR, of death 0.59, 95% CI 0.44 to 0.79) compared to only a 13% reduction in women with dense breasts (RR death 0.87, 95% CI 0.52 to 1.45). There is little benefit to adding mammography to MRI. This is particularly true for women under age 50, and the majority of cancers seen only on mammography are ductal carcinoma in situ. Appropriately, the EUSOBI guideline allows that MRI be performed either as a supplement to mammography or as a standalone examination.

What data underpin this recommendation? The ACRIN 6666 study showed that MRI depicted additional cancers in 9/612 (15/1000) women with dense breasts (heterogeneously dense or extremely dense) and elevated risk even after mammography plus ultrasound [5]. Kuhl et al [6] first showed that breast MRI greatly improves cancer detection after mammography in women of all breast densities at average risk (and women with dense breasts also had prior screening ultrasound). The incremental yield of MRI was 22.6/1000 (95%CI 17.4, 29.9) in the first year and 6.9 per 1000 in incidence screening rounds (95%CI 3.6, 9.9; 12–36 months later). In both these studies, the cancers seen by MRI were nearly all node negative.

The EUSOBI guideline cites two recent prospective multicenter trials that provide even stronger evidence of the performance of breast MRI in the broader population of women with dense breasts. In the ECOG-ACRIN 1141 randomized trial of abbreviated MRI vs. tomosynthesis, it is possible to examine performance of screening MRI alone in women with dense breasts. Among 1444 women, MRI alone depicted all 17 invasive cancers and 5/6 (83%) DCIS. Tomosynthesis alone depicted only 8/17 (47%) invasive cancers and 2/6 (33%) DCIS. In the Dutch DENSE trial (Dense Tissue and Early Breast Neoplasm Screening), women aged 50–75 with extremely dense breasts were invited to have biennial screening MRI after negative mammography. In the prevalence screening round, 79/4783 (16.5/1000) women were found to have cancer on MRI; 55/64 (86%) with invasive cancers staged were node negative. Another 375 women (80 per 1000) were recalled for false alarms. With incidence screening two years later, 20/3436 (5.8/1000) women were found to have cancer on MRI (including with 14 invasive cancer). With incidence screening MRI, there were fewer, 90/3416 (26 per 1000), false positive callbacks.

Since 2007, the American Cancer Society has recommended adding annual screening MRI to mammography for women at high risk [7]. In women with known pathogenic mutations, women screened with MRI have less advanced disease, fewer interval cancers, and more node-negative invasive cancers than those screened with mammography alone. The American College of Radiology and National Comprehensive Cancer Network also recommend annual screening MRI in women with a personal history of breast cancer diagnosed by age 50 or who have dense breasts [8]. These guidelines further suggest contrast-enhanced mammography or ultrasound be considered in women who qualify for screening MRI but are not able to tolerate it. In both the ACRIN 6666 study and the DENSE trial, 41–42% of women offered screening at MRI at no cost declined due to claustrophobia, fear of needles, inconvenience, and other reasons [9, 10].

EUSOBI suggests ultrasound for women who qualify for MRI but cannot tolerate or access it.

The EUSOBI guideline stops short of recommending additional screening the approximately 40% of to women with heterogeneously dense breasts. Screening ultrasound is much less effective than contrast-enhanced methods at depicting cancer. On average, incremental cancer detection rates for ultrasound are 2–3 per 1000 [11]. From 86 to 91% of cancers seen with screening ultrasound are invasive and about 90% are node negative [11]. Ultrasound does reduce interval cancer rates.

Contrast-enhanced mammography (CEM) performs similarly to MRI in depicting breast cancers due to increased and leaky blood vessels. Sensitivity of CEM appears comparable to MRI but there may be fewer false positives with CEM. In a meta-analysis of 13 diagnostic studies comparing CEM with MRI, Xiang et al [12] found overall sensitivity of each of CEM and MRI was 97%, but specificity was better with CEM. CEM has additional benefits compared to MRI, including lower cost, shorter acquisition time, and potentially increased access.

The EUSOBI guideline emphasizes the need for shared decision-making of patients and their healthcare providers. This requires both women and providers to be informed and for pertinent options to be available. While processes are still evolving, this major first step is to be applauded.