Breast cancer screening is associated with early detection and treatment as well as reduced morbidity and mortality from breast cancer. Screening practices have evolved from clinician-guided breast examination to imaging with mammography and ultrasonography.
Trials of the use of mammography pioneered by the Health Insurance Plan of New York project between 1967 and 1984 demonstrated improved detection rates. Mammography has since contributed significantly to the detection, measurement, and characterization of both palpable and nonpalpable breast lesions.
Increased breast density, a risk factor for breast cancer, hampers average screening practices. Dense breast tissue appears white on mammography, as opposed to the predominantly dark transparent image produced by nondense or fatty breast tissue. The images produced by dense breasts may therefore mask abnormal breast tissue changes, such as tumors or calcification, which also appear white. These cases require a more tailored screening approach that may include alternative or additional screening with ultrasound or magnetic resonance imaging (MRI).
The National Comprehensive Cancer Network (NCCN) and the American Cancer Society recommend supplementary MRI screening for some women at increased risk for breast cancer. However, existing guidelines cite insufficient evidence for a recommendation of MRI in women with dense breasts.
The Dense Tissue and Early Breast Neoplasm Screening (DENSE) investigators examined whether supplemental MRI screening in women with dense breasts improved early detection and reduced interval breast cancers. The results indicated that supplemental MRI screening in women with extremely dense breast tissue and normal results on mammography was associated with the diagnosis of significantly fewer interval cancers than mammography alone, but also led to more benign biopsies.
The following NEJM Journal Watch summary explains the study and results further:
Do Women with Extremely Dense Breasts Benefit from Supplemental MRI Screening?
Laila Samiian, MD, FACS reviewing Bakker MF et al. N Engl J Med 2019 Nov 28
Breast MRI detected more cancers but also led to more biopsies that were benign.
Women with extremely dense breast tissue have excess risk for developing breast cancer; however, no specific guidelines address the evaluation of such women. In a Dutch trial, 40,000 women (age range, 50–75) with extremely dense breasts and a recent negative screening mammogram were randomized to undergo — or not undergo — supplemental breast MRI.
Among women assigned to breast MRI, 59% actually underwent MRI imaging; among those women, 95 per 1000 were recalled for further evaluation, and breast cancer was detected in 16 per 1000. MRI-detected cancers were more likely to be small and categorized as ductal carcinoma in situ. The rate of interval cancers within 2 years (the interval before the next routine biennial mammogram) was significantly lower in the breast MRI group than in the mammography-alone group (2.5 vs. 5.0 per 1000 screens). The positive predictive value of MRI was 17%, and 74% of biopsies ordered based on MRI findings were benign.
Comment: These results confirm that breast MRI is more sensitive than mammography for detecting breast cancer in women with extremely dense breasts; however, this approach has low specificity and can lead to benign biopsies. Furthermore, it's not clear whether supplemental imaging can reduce breast cancer mortality; without long-term follow-up, incidence of interval cancer serves as a surrogate for mortality. Cost-effectiveness, risk for over-diagnosis and treatment of nonlethal indolent cancers, potential harms of unnecessary biopsies, and patients' anxiety all must be considered. Lastly, not all women with dense breasts as their only risk factor will necessarily benefit from an MRI; individualized risk assessment might be better for identifying high-risk women (lifetime risk >20%) who would benefit from MRI screening.
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Krista is a 2019-2020 editorial fellow at the New England Journal of Medicine. She is from Nassau, Bahamas where she is training in general surgery at the University of the West Indies.