Annual mammograms for women aged 40 to 59 do not reduce mortality from breast cancer and may result in overdiagnosis, according to study published in the BMJ. However, the American College of Radiology (ACR) questioned the study’s findings and conclusions.
As the debate of how frequently women who are at average risk of developing breast cancer continues, Canadian researchers reported on a 25-year long randomized screening trial that involved 89,835 women, aged 40 to 59, from six Canadian provinces.
The study participants were randomized to either undergo five mammography screens, one per year for five years (44,925 participants), or no mammograms (44,910 participants). All of the women aged 40 to 49 in the mammography arm and all women aged 50 to 59 in both arms received annual physical breast examinations. Those aged 40 to 49 who were in the control arm received a single examination followed by usual care in the community.
The researchers found that during the five-year screening period, 666 invasive breast cancers were diagnosed in the mammography arm and 524 in the control group. Of these, 180 women in the mammography arm and 171 women in the control arm died of breast cancer during the 25-year follow-up period.
The findings for women aged 40 to 49 and 50 to 59 were almost identical. During the entire study period, 3,250 women in the mammography arm and 3,133 in the control arm had a diagnosis of breast cancer, and 500 and 505, respectively, died of breast cancer.
“After 15 years of follow-up a residual excess of 106 cancers was observed in the mammography arm, attributable to over-diagnosis,” the authors wrote.
The authors concluded that while education, early diagnosis, and excellent clinical care should continue, “Annual mammography in women aged 40 to 59 years did not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available. Overall, 22 percent (106/484) of screen detected invasive breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.”
In an accompanying editorial, Mette Kalager, MD, and colleagues believe that long-term follow-up does not support screening women under 60. They agree with the study authors that "the rationale for screening by mammography be urgently reassessed by policy makers," but point out that this is not an easy task "because governments, research funders, scientists, and medical practitioners may have vested interests in continuing activities that are well established."
The ACR, however, disputed the study findings and the accompanying editorial. The organization called the study “an incredibly misleading analysis based on the deeply flawed and widely discredited Canadian National Breast Screening Study (CNBSS).”
“The results of this BMJ study, and others resulting from the CNBSS trial,” the group said in a statement, “should not be used to create breast cancer screening policy as this would place a great many women at increased risk of dying unnecessarily from breast cancer.”
Experts who reviewed the study’s mammograms reported that they were of poor quality and the mammography machines used for the study were not considered to be state-of-the-art for the time. It is noted that the low number of cancers detected by mammography was consistent with poor quality mammography.
“The images were compromised by ‘scatter,’ which makes the images cloudy and cancers harder to see since they did not employ grids for much of the trial,” the ACR stated. The group also noted that the CNBSS radiologists had not been trained for mammographic interpretations.
Other areas of concern were raised by the ACR, including that the patients were not truly randomized and the investigators were made aware of which patients had been diagnosed with incurable cancer.
Considering these factors, the ACR does not believe that the CNBSS should be considered as a legitimate trial of modern screening mammography.