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, according to findings published online on Sept. 22 in the New England Journal of Medicine.
“The use of screening mammography is still debated, chiefly because of concern regarding methodologic limitations in some of the randomized trials,” wrote Dr. Mette Kalager of the Cancer Registry of Norway, Oslo, and the Harvard School of Public Health in Boston, and colleagues. 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 rate of death in the screened group of women aged 50-69 years was 18 per 100,000 person-years, compared with 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, Dr. Kalager and associates noted.
Part of the reduction in both screened and unscreened groups was “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.
“To our surprise, the reduction in breast cancer mortality among women [aged 70-84] was largely the same as that in the screening group,” they added.
The study was limited by a relatively short follow-up period and by the possibility that some women in the unscreened group may have in fact been screened. The results suggest that screening mammography does reduce the rates of death from breast cancer, but the benefits may occur only in the context of “a well-functioning health care system that is available to the entire population,” the investigators said.
The study was funded by the Cancer Registry of Norway and the Research Council of Norway. Dr. Kalager and associates had no financial conflicts to disclose. Dr. Welch had no relevant financial disclosures.