The study compared the number of invasive breast cancers in two nearly identical groups-each with about 100,000 Norwegian women aged 50 to 64. The first group was followed from 1992 to 1997; the second from 1996 to 2001. The key difference between the two periods is that before 1996, Norway had no national breast-cancer screening program, while after 1996, all women aged 50 to 69 were offered a mammogram every two years. So the women in the first group had only one mammogram, at the end of that six-year span. But the women in the second group (the screened group) had three mammograms during that six-year span.
Given the conventional thinking about invasive breast cancer-that it always progresses-one would expect that the total number of cancers detected in each group at the end of six years would be about the same. It wasn't. The incidence of invasive breast cancer was 22% higher in the screened group. (emphasis mine - Kristi)
... The USPSTF reasoned that the additional benefit gained by starting screening at age 40 years rather than at age 50 years is small, and that moderate harms from screening remain at any age. This leads to the ... recommendation against routine screening of women aged 40 to 49 years.... The decision to start regular, biennial screening mammography before the age of 50 years should be an individual one and take patient context into account, including the patient’s values regarding specific benefits and harms...
Recent appreciation of the frequency of premalignant, in situ, and even indolent low-grade cancers that will never progress to symptomatic or life-threatening disease has influenced everyone’s approach to cancer screening (43,44,62,63). Recent estimates are that up to “one in three breast cancers detected in a population offered screening is over- diagnosed” (62). The analogy between the overdiagnosis of breast cancer with mammographic screening and that of prostate cancer with prostate-specific antigen screening is well known. As one researcher observed, “There are many studies showing that mammograms find low-risk, well-behaved cancers preferentially and miss the bad actors preferentially, [with] paper after paper saying mammograms pick up cancers that don’t need to be found” (45).
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