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Breast Density and Risk of Interval Cancers.

Breast cancer screening in Canada varies by province. The Canadian Task Force on Preventive Health Care recommends against routine screening for women aged 40-49 at average risk, in spite of evidence that the most lives are saved by annual screening starting at age 40. Each province chooses what age women can attend screening mammography and at what interval: eight provinces/territories start screening at age 50, but some of those allow women to attend starting at 40 if they have a requisition. Four allow women to self-refer (without a physician’s requisition) starting at age 40. Most provinces screen biennially, but some allow all women to attend annually from 40-49. Not included in the analysis: all women in BC could self-refer to screening annually during the period of the study. Yukon is currently the only jurisdiction where all women may self-refer annually from 40-74. Density reporting is also variable: some provinces use the ACR BIRADS 5th edition 4-category density scoring system; some use a binary system, usually less than or greater than 75% (based on BIRADS 4th edition categories). Thanks to patient advocacy, there is increasing notification of women of their breast density, but not yet all across Canada. Some provinces designate only women with ACR BIRADS category D as having dense breasts. Supplemental screening ultrasound for women with dense breasts is theoretically covered by public health insurance in all jurisdictions. BC is the only jurisdiction that explicitly covers screening ultrasound for all women with categories C and D. In this paper, Seely et al focus on the critical relationship of breast density to interval cancers. They point out that in 5 jurisdictions, women are screened annually if they have category D density breasts. Mammography doesn’t detect all cancers. The undetected cancers can grow and are frequently found by women as a lump in the interval between planned screening visits, and these are called interval cancers. Some interval cancers that are easily seen mammographically at the time they are clinically detected, can be shown to have not been present on the prior screen, even in non-dense tissue. This scenario suggests a rapidly-growing tumour that arose de novo after the prior examination. Cancers may be undetected if they are masked in normal dense tissue. Mammography sensitivity decreases, as mammographic breast density increases, and mammograms miss up to 50% of cancers in women with the densest breasts. So, women with dense breasts are more likely to have interval cancers. In addition to the masking of cancer, dense breast tissue is an independent risk factor for developing cancer. Interval cancers are, on average, larger and more often nodepositive at the time of diagnosis, and frequently higher grade. They have a poorer prognosis than screen-detected cancers. Hence, one of the goals of screening is to reduce interval cancers, and this can be achieved by more effective screening: by screening more frequently and/or by supplemental screening with other imaging modalities. Our national patchwork of breast cancer screening policies provides a natural experiment to study the relationship between screening frequency and the incidence of interval cancers. Seely et al compared the incidence of interval cancer in provinces where mammography is performed biennially, to provinces that recall women with dense breasts annually. They showed significantly fewer interval cancers in provinces where annual mammography is done for women with dense breasts. This important information should prompt all jurisdictions to recall women with dense breasts annually, allowing some degree of personalization of screening recommendations. It partially addresses the inequity in breast cancer screening for women with dense breasts, who do not benefit from mammography to the same extent as women with non-dense breasts. Others have shown that breast density is significantly associated with the diagnosis of interval cancer versus screen-detected cancer, for either visual BIRADS assessment or automated volumetric breast density. This gives reassurance that until such time that all provincial screening programs institute automated density assessment, visual density assessment will suffice. Breast density is important health information, and education of referring physicians and women about breast density is needed. Women with dense breasts should understand the associated risks, and deserve to be told their breast density in their screening mammography report. It’s not sufficient that the information be shared only with their physician. Most importantly, they deserve equitable access to early detection of breast