SAN FRANCISCO – MRI-screening may improve the detection of biologically relevant breast cancer in women who are at average-risk, and reduce the interval-cancer rate down to 0%, at a low false-positive rate, say the authors of a new study presented at the 2015 Breast Cancer Symposium.
In this cohort of heavily pre-screened women at average risk, the additional cancer yield achieved through MRI was high, at 15.8 cases per 1,000 women screened, and the added cancers diagnosed by MRI tended to be of high nuclear grade.
Dr. Christiane Kuhl
“MRI improves the detection of small high grade cancers in women at average risk, to the extent that the interval cancer rate was zero,” said Dr. Christiane Kuhl, of the University of Aachen, Germany.
“Between 30% and 50% of cancers identified in women who undergo screening mammography are not detected by mammography,” she said during her presentation.
Breast cancer continues to be the main or second most important cause of cancer death in women, and is the main cause of life years lost in women. “Which tells you that the main problem with mammography screening is not overdiagnosis,” she said. “The main problem is actually underdiagnois of disease.”
Breast MRI is currently recommended for screening women who are at a high-risk of breast-cancer, but despite decades of mammographic screening, breast cancer continues to cause high mortality for women who are deemed to be at an average risk of the disease. These data suggest that there is a need for improved methods of early diagnosis in this population.
Dr. Kuhl and her colleagues investigated the utility of supplemental MRI-screening of women who carry an average-risk of breast-cancer, and they conducted a prospective observational cohort-study that assessed the use of MRI screening in this population. The participants were in the usual age range for screening-mammography (40-70 years).
The women underwent dynamic contrast-enhanced MRI of the breast in addition to mammography every 12, 24, or 36 months, plus follow-up of 2 years to establish a standard-of-reference. The cohort included 2,120 women who underwent a total 3,861 MRI screenings.
Breast-cancer was diagnosed in 61 women (DCIS: 20, invasive: 41), and the rate of interval cancers was 0%, irrespective of screening interval. Of these women, 48 were diagnosed at prevalence-screening by MRI alone (supplemental cancer detection rate: 22.6 cases per 1,000 women screened. In addition, 13 women were diagnosed with breast-cancer in 1,741 incidence-screening-rounds collected over 4,887 women-years. A total 12 of these 13 incident cancers were diagnosed by screening-MRI alone (supplemental cancer detection rate was 6.9 per 1,000), one by MRI and mammography, and none were by mammography alone.
The authors also found that the supplemental cancer detection rate was independent of mammographic breast density. Invasive cancers were small, with a mean size of 8mm. They were node-negative in 93.4%, ER/PR-negative in 32.8%, and de-differentiated in 41.7% at prevalence, and 46% at incidence-screening. The specificity of MRI-screening was 97.1%, while false positive rate was 2.9%.
In a discussion of the paper, Dr. A. Marilyn Leitch, of the University of Texas Southwestern Medical Center, Dallas, pointed out that this was a selected population with negative mammograms. But is it possible to “apply MRI at longer intervals, say every 3 years, without initial mammography as a screening tool, in average risk women?” she asked
One of her concerns, however, is that “we hear all the time about the high false positive rates with MRI,” and they are generally higher than was reported in this study. “MRI also detects lesions that might drive patients to avoid breast conserving surgery, and there can be unreasonable costs for screening and work up of false positives.”
Screening guidelines from the USPSTF say that evidence for replacing mammography with MRI is lacking and the balance of benefit versus harms cannot be determined. “Even more ‘liberal’ guidelines from the American Cancer Society reserve MRI for high risk patients.”
What is needed is a clinical trial, she summarized, that will compare screening MRI at 3 year intervals to tomosynthesis mammogram annually in average risk women.