(HealthDay News) — Most breast cancer deaths occur in unscreened women, according to a study published online Sept. 9 in Cancer.

To investigate whether breast cancer deaths predominantly occur in unscreened women, Matthew L. Webb, from Massachusetts General Hospital in Boston, and colleagues followed cases of invasive breast cancers diagnosed between 1990 and 1999 through 2007. Demographics, mammography use, surgical and pathology reports, recurrence, and death dates were evaluated. Medical records were used to characterize mammograms as screening or diagnostic based on absence or presence of breast signs or symptoms.

The researchers found that invasive breast cancer failure analysis defined 7,301 patients between 1990 and 1999, which included 1,705 documented deaths from breast cancer (609 deaths) or other causes (905 deaths). Twenty-nine percent of the confirmed breast cancer deaths were among women who had been screened (19% screen-detected and 10% interval cancers).

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Unscreened women accounted for 71% of the breast cancer deaths, including those whose most recent mammogram was more than 2 years before diagnosis (6%) and those who were never screened (65%). For fatal cancers the median age at diagnosis was 49 years, whereas the median age at diagnosis was 72 years for deaths not from breast cancer.

“To maximize mortality reduction and life-years gained, initiation of regular screening before age 50 years should be encouraged,” the authors write.