A previous study by the authors found the increase in breast cancer incidence among young women couldn't be attributed to the beginning of screening mammography in the 1970s. The findings of the new study also showed the change isn't tied to having a larger population size.
"This study found that breast cancer incidence has been significantly increasing for the past 80 years, with the increase beginning at least a decade before the baby boom," wrote the authors, led by Sarah Lima from the Mailman School of Public Health at Columbia University.
For their study, the researchers looked at breast cancer cases and parity, a term that refers to the number of live births in a population. They wondered whether population changes could explain a previously noted increase in breast cancer cases.
To find out, the researchers analyzed annual age-adjusted breast cancer incidence rates for women in Connecticut from 1935 to 2015. Their data came from the National Vital Statistics System and the Connecticut Tumor Registry, which are among the oldest cancer registries in the U.S.
For women ages 25 to 39, the number of breast cancer diagnoses per 100,000 people rose from 16.3 in 1935 to 38.5 in 2015, representing a 0.65% annual percentage change and 2.4% total increase. The rise in cancer cases remained significant even after the researchers controlled for changes in the fertility rate.
"Although recent parity trends track with breast cancer incidence trends and can partially explain the increased incidence, they do not explain most of the increase, and the historical trends using a much longer time lens provide a strong argument against parity being the primary factor associated with increasing breast cancer incidence, particularly in young women," the authors wrote.
The number of breast cancer diagnoses per 100,000 people also drastically rose for women ages 40 and older between 1935 and 2015. However, the finding fell out of significance once the researchers adjusted for population growth, decline, and stabilization.
The data also confirmed the researchers' previous findings that mammography can't explain the change in breast cancer incidence. For one, the majority of women under the age of 40 don't participate in routine mammography. Furthermore, the increase in breast cancer incidence began before routine mammography was implemented in the 1970s, the authors noted.
"This result is important because it establishes upward trends in breast cancer incidence prior to the 1970s," the authors wrote. "We also found the increase in breast cancer incidence began more than three decades before the secular decrease in parity; thus, the increase in breast cancer cannot be attributed to declining trends in parity."
The authors cautioned that they only had national birth data for their analysis, so they couldn't specifically look at parity data for Connecticut. In addition, Connecticut has a higher breast cancer rate than the U.S. as a whole, but they believe the strength of having so much retrospective data makes up for the limitations.
"While this may reduce the generalizability of our study, we believe the importance of using data available prior to the baby boom outweighs concerns of external validity," the authors wrote.
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