Next they looked at the types of cancers that were diagnosed. As expected, cases of stage 0 (also known as ductal carcinomas in situ, or DCIS) and stage 1 breast cancers "increased sharply," they wrote. But these extra early-stage cancers were not offset by a decline in later-stage cancers; instead, the incidence of stage 2, stage 3 and stage 4 breast cancers "remained relatively stable," they found.
If screening wasn't working as expected, why were there fewer deaths? The researchers extrapolated data from before the screening program began to estimate the effect of improved cancer treatments.
In their first scenario (which did not include cohort effects), better treatments were linked with a 28 percent decline in breast cancer deaths and screening mammograms got credit for a 5 percent reduction. That means 85 percent of the observed decrease in breast cancer deaths since the screening program was due to better treatment, not the program itself.
When cohort effects were added back into the mix, the effect of better treatments didn't change but the benefits of screening were erased.
"After 23 years of screening, the reduction in breast cancer mortality in women aged 50 or more associated with screening ranges from 0 to 5 percent," the researchers concluded.
Deaths also fell for women under 50 -- who were too young to be screened -- and that decline was not linked to a change in the incidence of later-stage breast cancers. The researchers called this further evidence that the reduction in cancer deaths "entirely resulted from improved treatments."
If the screening mammograms were merely useless, the authors might have lamented the loss of resources that could have been channeled toward some other set of medical problems. But the screening program didn't just fail to help; it caused harm to women who were treated for cancers that were harmless and could have been safely ignored.
The researchers estimated that 33 percent of the stage 0 cancers found on digital mammograms were cases of "overdiagnosis." So were 59 percent of the stage 1 cancers.
The findings line up with anecdotal evidence that the screening program isn't working as intended. The researchers noted that breast cancer mortality in Belgium fell by 34 percent during the period when the Dutch saw a 38 percent decline, even though the Belgians began their nationwide screening regimen 15 years later.
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