A study by Welch et al published in New Engl J Med added to the growing evidence that for every woman who was helped by screening mammography many have been harmed.
The authors used data from the SEER program from 1975 through 2012. They calculated the size-specific cancer case fatality rate in women 40 year old and older for two time periods: a baseline period before the implementation of widespread screening mammography (1975 through 1979) and a period encompassing 10 years (2000 through 2002).
Screening did result in more cancers being detected, with invasive tumors measuring <2 cm or in situ carcinomas increased from 36% to 68%; and the detected tumors that were large, which were invasive cancers and measuring ≥2 cm decreased from 64% to 32%. Surprisingly though the data suggest that only about 30 of the 162 additional small tumors per 100,000 women that screening mammograms found would ever have progressed to a dangerous stage. That means that 132, or 81 percent, of the 162 extra tumors detected represented overdiagnosis, that resulted to treatment of tumors that were never destined to harm.
Most importantly, the incidence of metastatic cancer, which is the type that causes most deaths, was flat.
In conclusion the findings from this study indicate that screening mammography finds many small cancers the dogma that all will grow, metastasize resulting in fatality is questionable. The authors also suggest that although screening may decrease cancer mortality as reflected by the declining incidence of larger tumors, they believe that the two thirds reduction in breast cancer mortality is due to recent advances in treatment.
N Engl J Med 2016; 375:1438-1447
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