Study contributes valuable data, but we need more on long-term outcomes
The trial by Bakker and colleagues employed a solid study design as women were randomly assigned to supplemental MRI screening or ongoing biennial mammography, and nearly all cancers were identified in the short-term of follow-up. In addition, very few women were lost to follow-up, and secondary outcomes, including false-positive rates, were collected to help providers and patients better understand some of the potential downsides of supplemental screening.
The substantial reduction in interval cancers (50% in the intent-to-screen analysis and 84% in the women who actually underwent supplemental MRI) was highly statistically significant (P<.001). While there were substantially fewer interval cancers in the MRI-assigned group, the interval cancers that did occur were of similar stage as those in the women assigned to the mammography-only group (TABLE 2).
Data demonstrate that interval cancers appear to be more aggressive than screen-detected cancers.4 While reducing interval cancers should be a good thing overall, it remains unproven that using supplemental MRI in all women with dense breasts would reduce breast cancer specific mortality, all-cause mortality, or the risk of more invasive treatments (for example, the need for chemotherapy or requirement for mastectomy).
On the other hand, using routine supplemental breast MRI in women with extremely dense breasts would result in very substantial use of resources, including cost, radiologist time, provider time, and machine time. In the United States, approximately 49 million women are aged 50 to 74.5 Breast MRI charges commonly range from $1,000 to $4,000. If the 4.9 million women with extremely dense breasts underwent supplemental MRI this year, the approximate cost would be somewhere between $4.9 and $19.5 billion for imaging alone. This does not include callbacks, biopsies, or provider time for ordering, interpreting, and arranging for follow-up.
While the reduction in interval cancers seen in this study is promising, more assurance of improvement in important outcomes—such as reduced mortality or reduced need for more invasive breast cancer treatments—should precede any routine change in practice.
Unanswered questions
This study did not address a number of other important questions, including:
Should MRI be done with every round of breast cancer screening given the possibility of prevalence bias? Prevalence bias can be defined as more cancers detected in the first round of MRI screening with possible reduced benefit in future rounds of screening. The study authors indicated that they will continue to analyze the study results to see what occurs in the next round of screening.
Is there a similar impact on decreased interval cancers in women undergoing annual mammography or in women screened between ages 40 and 49? This study was conducted in women aged 50 to 74 undergoing mammography every 2 years. In the United States, annual mammography in women aged 40 to 49 is frequently recommended.
What effect does supplemental MRI screening have in women with heterogeneously dense breasts, which represents 40% of the population? The US Food and Drug Administration recommends that all women with dense breasts be counseled regarding options for management.6
Do these results translate to the more racially and ethnically diverse populations of the United States? In the Netherlands, where this study was conducted, 85% to 90% of women are either Dutch or of western European origin. Women of different racial and ancestral backgrounds have biologically different breast cancers and cancer risk (for example, higher rates of triple-negative breast cancers in African American women; 10-fold higher rates of BRCA pathogenic variants in Ashkenazi Jewish women).
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