The availability of screening mammography accounted for a 10% relative reduction in deaths from breast cancer from 1996 through 2005, based on data from more than 40,000 women with breast cancer.
“The use of screening mammography is still debated, chiefly because of concern regarding methodologic limitations in some randomized trials,” Dr. Mette Kalager of the Cancer Registry of Norway, Oslo, and the Harvard School of Public Health in Boston, and colleagues reported.
Norway implemented a nationwide breast cancer screening program in 1996. To avoid some of the limitations of previous studies, the researchers divided 40,075 women with breast cancer into four groups: those in counties of Norway with and without breast cancer screening programs between 1996 and 2005, and two historical comparison groups of women living in these same areas between 1986 and 1995. The researchers obtained information on breast cancer as the cause of death through links between the Cancer Registry of Norway and the Cause of Death Registry at Statistics Norway (N. Engl. J. Med. 2010;363:1203-10).
Women who were aged 50-69 years beginning in 1996 were eligible for screening mammography. The maximum follow-up time was 8.9 years. Overall, 4,791 (12%) of the women with a breast cancer diagnosis died, and 423 of these women (9%) were diagnosed after the introduction of the screening program.
The death rate in the screened group of women aged 50-69 years was 18 per 100,000 person-years, vs. 25 per 100,000 person-years in their historical counterparts. The rate of death in the unscreened group was 21 per 100,000 person-years, compared with 26 per 100,000 person-years in their historical counterparts.
These numbers translate to a 28% drop in breast cancer mortality in the screened group and an 18% drop in the unscreened group, compared with their historical counterparts, suggesting a 10% relative reduction in mortality from breast cancer screening alone. Part of the reductioniwas “presumably a result of increased breast cancer awareness, improved therapy, and more sensitive diagnostic tools,” they said.
When mortality rates were broken down by stage, women in the screened group with stage I tumors had a 16% relative reduction in mortality, compared with their historical counterparts. Women in the unscreened group had a 13% relative reduction in mortality, compared with their historical counterparts.
Women in the screened group with stage II tumors had a 29% reduction in mortality, compared with their historical counterparts. The reduction in mortality in the unscreened group was 7%. Women with stage III or IV tumors showed equally reduced mortality from cancer in both the screened and unscreened groups (rate ratio for death in both groups, 0.70), compared with their historical counterparts.
Women who were younger than 50 years or older than 69 years and therefore not eligible for screening during the study period also showed fewer deaths from breast cancer per 100,000 person-years, compared with their historical counterparts. Women in these age groups likely benefited from the presence of multidisciplinary cancer care teams, although they were not screened for breast cancer, the researchers noted.
However, “the reduction in breast cancer mortality among women [aged 70-84] was largely the same as that in the screening group,” they added.
The Cancer Registry of Norway and the Research Council of Norway funded the study. Dr. Kalager and associates had no financial conflicts to disclose.
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The Impact of Screening Mammography has Declined
Dr. H. Gilbert Welch noted that the 10% reduction in death rates in the study by Dr. Kalager and colleagues is below the 15%–23% reduction estimated by the U.S. Preventive Services Task Force in a study published in 2009 (Ann. Intern. Med. 2009;151:738-42).
Dr. Welch commented that, based on the historical comparisons used in the study, “it is quite plausible that screening mammography was more effective in the past than it is now.”
He suggested that increased awareness of breast cancer and of the need to seek care for overt breast abnormalities have made screening less of a factor in reducing breast cancer deaths.
He also emphasized that the reduction in mortality in this study appeared to be due to a combination of both screening and the multidisciplinary teams that provided better breast cancer treatment.
Indeed, the study provides data that the treatment may be most important, since women over age 70 years who were not offered screening mammography had an 8% reduction in breast cancer mortality.
“Thus, the relative reduction in mortality due to screening mammography alone could be as low as 2%,” he said.
Dr. Welch also raised the issue of the false alarm.