52% of the late-stage breast cancer cases were associated with an absence of screening

Efforts focused on increasing the use of screening mammography among targeted groups of women should be made a top priority to achieve the largest reduction in late-stage breast cancers, according to the authors of a new study in the October 20 issue of the Journal of the National Cancer Institute.

However, the author of an editorial in the same issue suggests that certain biases often found in breast cancer screening studies should be taken into account when interpreting the results of the study.

Because mammography increases the detection rate of early-stage breast cancers, the incidence of late-stage breast cancers should decline. However, despite high levels of screening in the general population, late-stage breast cancers still occur. The occurrence of late-stage breast cancers could indicate failures in the screening process.

To establish where the screening process breaks down and where changes in care might have the greatest impact, Stephen H. Taplin, M.D., M.P.H., now at the National Cancer Institute, and colleagues in a consortium of integrated health plans, The Cancer Research Network, conducted a retrospective case–control study using data from seven health care plans that included 1.5 million women aged 50 years and older who had access to screening mammography. They compared women who had been diagnosed with late-stage breast cancer with those who had been diagnosed with early-stage breast cancer and, on the basis of their earliest screening mammogram during the period reviewed, categorized the women into one of three groups: absence of screening, absence of detection, or potential breakdown in follow-up.

The authors found that 52% of the late-stage breast cancer cases were associated with an absence of screening, 39% with an absence of detection, and 8% with a potential breakdown in follow-up. They also found that the odds of having late-stage cancer were higher among women with an absence of screening. Among women diagnosed with late-stage cancer, women were more likely to be in the absence-of-screening group if they were aged 75 years or older, unmarried, or did not have a family history of breast cancer. In addition, women who had less education or lower income were more likely to have been in the absence-of-screening group.

"To reduce late-stage cancers, priority should be given to promoting screening among those women without a mammogram within 2 years and improving breast cancer detection at the time of screening," the authors write. "Top priority, however, should be given to reaching unscreened women, especially those who are likely to be older, to have a low annual income, and to have less education, even in organized health plans."

In the editorial, Michael Baum, M.D., of University College London, takes issue with this interpretation of the data and discusses three common sources of possible bias that apply to breast cancer screening. He notes that the effect of increasing the number of women who receive mammograms would be limited because the incidence of interval cancers--cancers diagnosed after a negative screening mammogram--would not change. "[E]fforts directed at improving the socioeconomic status of women in lower strata might indirectly have a greater effect on reducing breast cancer mortality than efforts directed at attaining universal mammography," he writes.