The incidence of late-stage breast cancers has decreased by 37 percent since the advent of mammography 30 years ago, according to a study published online June 10 in Cancer.

“While we have seen an increase in overall breast cancer incidence over the last 30 years, the drop in late-stage diagnoses is a positive benefit of mammography and our heightened awareness of early detection,” said lead author Mark Helvie, MD, of the University of Michigan Comprehensive Cancer Center in Ann Arbor, in a press release. “The decrease in late-stage disease, together with improved treatments, contributes to the decreased mortality from breast cancer in the United States in the last 20 years.”

Helvie and colleagues conducted an analysis of early and late-stage breast cancer diagnoses between 1977 and 1979, which was before mammography became popular, and compared it to diagnoses between 2007 and 2009. Given the trends seen in the pre-mammography period and over time, the researchers considered a central estimated increase in breast cancer incidence of 1.3 percent per year, which is designated as the annual percentage change (APC).

After examining the late 1970s data and projected incidence of early-stage and late-stage breast cancer in 2007 to 2009 based on the APC, they compared the projected rates to actual rates.

Results indicated that late-stage breast cancer incidence decreased by 37 percent from the projected rate, while early-stage breast cancer incidence increased 48 percent, indicating a shift toward earlier detection over the study period. Even if APC estimates were adjusted down to 0.5 percent or up to 2 percent, the analysis still revealed a substantial decrease in observed late-stage disease compared with projections.

Since mammography was introduced, there has been an overall nine percent decrease in invasive breast cancer when factoring in a 1.3 percent APC.

“This is what you would expect with a successful screening program,” said Helvie. “Not only are we detecting more early-stage cancer, but we are decreasing the number of late-stage cancers that tend to be more challenging to treat and more deadly.”