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For the study, researchers examined data from the National Cancer Institute collected from 1975 through 2012, to calculate the number of breast cancer findings and the size of tumors among women age 40 or older. They then calculated the size-specific cancer case fatality rate before the implementation of widespread screening mammography (1975 through 1979) and a period encompassing the most recent years for which 10 years of follow-up data were available (2000 through 2002).
According to the Los Angeles Times, the study suggests that the majority of abnormalities found through screening mammograms would likely never become life-threatening if left alone.
After the advent of screening mammography, the proportion of detected breast tumors that were small increased from 36 percent to 68 percent; the proportion of detected tumors that were large decreased from 64 percent to 32 percent, the study found.
“However, this trend was less the result of a substantial decrease in the incidence of large tumors (with 30 fewer cases of cancer observed per 100,000 women in the period after the advent of screening than in the period before screening) and more the result of a substantial increase in the detection of small tumors (with 162 more cases of cancer observed per 100,000 women),” the study’s author’s wrote.
Researchers estimated that if the underlying disease burden was stable for the course of the study, only 30 of the 162 additional small tumors detected per 100,000 would have become large, which implied the remaining 132 cases of cancer per 100,000 women were overdiagnosed.
“The potential of screening to lower breast cancer mortality is reflected in the declining incidence of larger tumors. However, with respect to only these large tumors, the decline in the size-specific case fatality rate suggests that improved treatment was responsible for at least two thirds of the reduction in breast cancer mortality,” the authors wrote.
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