I was in bed, ready to go to sleep when I saw this study. It upset me so much I had to get up to write about it. From the BMJ:
Objective: To compare breast cancer incidence and mortality up to 25 years in women aged 40-59 who did or did not undergo mammography screening.
Design: Follow-up of randomised screening trial by centre coordinators, the study’s central office, and linkage to cancer registries and vital statistics databases.
Setting: 15 screening centres in six Canadian provinces,1980-85 (Nova Scotia, Quebec, Ontario, Manitoba, Alberta, and British Columbia).
Participants: 89 835 women, aged 40-59, randomly assigned to mammography (five annual mammography screens) or control (no mammography).
Interventions: Women aged 40-49 in the mammography arm and all women aged 50-59 in both arms received annual physical breast examinations. Women aged 40-49 in the control arm received a single examination followed by usual care in the community.
Main outcome measure: Deaths from breast cancer.
Results; During the five year screening period, 666 invasive breast cancers were diagnosed in the mammography arm (n=44 925 participants) and 524 in the controls (n=44 910), and 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 overall hazard ratio for death from breast cancer diagnosed during the screening period associated with mammography was 1.05(95% confidence interval 0.85 to 1.30). The findings for women aged 40-49 and 50-59 were almost identical. During the entire study period, 3250 women in the mammography arm and 3133 in the control arm had a diagnosis of breast cancer, and 500 and 505, respectively, died of breast cancer. Thus the cumulative mortality from breast cancer was similar between women in the mammography arm and in the control arm (hazard ratio 0.99, 95% confidence interval 0.88 to 1.12). After 15 years of follow-up a residual excess of 106 cancers was observed in the mammography arm, attributable to over-diagnosis.
Conclusion: Annual mammography in women aged 40-59 does 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%(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.
More than twenty-five years ago, 89,835 women between the ages of 40-59 were randomized to either get annual screening with mammography for five years or no mammography. I can imagine some of you are gasping in horror. After all, how could we possible not give women annual mammograms. Wouldn’t that be malpractice?
Of the 44,925 women in the mammogram group, 500 died of breast cancer. Of the 44, 910 in the no mammogram group, 505 died of breast cancer. This was not a significant difference. There wasn’t a significant difference if you looked at only older women (50-59) or younger women (40-49). There wasn’t a difference if you lengthened the screening period to seven years.
Mammograms did not affect mortality at all.
However, they did affect diagnosis. During the screening period, 666 cases of cancer were diagnosed in the mammography group versus 524 in the no mammography group. This meant an excess of 143 breast cancers were diagnosed with screening. Fifteen years later, the excess settled in at 106 cases of cancer.
More than 20% of the cancers detected by mammography were over-diagnosed. This means that mammography over-diagnosed one case of breast cancer for every 424 women screened with mammography. Do you know how many women we screen a year here?
This study is going to make a whole lot of people upset. It’s a large, well designed randomized control trial with a really long follow-up period. The people in the mammogram groups actually complied with screening in surprisingly high numbers. It’s hard to find fault with much of this. The data make a really good case that universal screening with mammograms does almost no good, and likely does harm.
Oh, they did find that the 25-year survivial rate was significantly better in the mammogram group (even if mortality wasn’t). But you all know why, right?
Although the difference in survival after a diagnosis of breast cancer was significant between those cancers diagnosed by mammography alone and those diagnosed by physical examination screening, this is due to lead time, length time bias, and over-diagnosis.
I wish everyone would watch this:
