Study shows benefit of mammography screening in women age 40-49

Mammography screening in women age 40 to 49 reduces breast cancer mortality rates, according to a new study.

Between 1986 and 1997, based on national recommendations, counties in Sweden began inviting women age 40 to 74 to mammography screening for breast cancer. Women age 40 to 54 were invited to screening every 18 months, while women age 55 to 74 were invited to screening every other year. The recommendations were modified in 1987 and 1988, allowing counties with fewer resources to focus screening on women age 50 to 74. As a consequence, about half of the counties screened women beginning at age 40 and half screened women beginning at age 50.

Researchers performed a study to compare death from breast cancer in women age 40 to 49 in areas that did and did not invite this age group to screening from 1986 to 2005. The authors also defined a prescreening reference period from 1970 to 1985, before organized screening began.

The study's main outcome measure was "refined mortality," defined as women age 40 to 49 who received a diagnosis of breast cancer and died of the disease during follow-up. The results were presented last week at the 2010 Breast Cancer Symposium and were published online by Cancer.

The study's average follow-up was 16 years. No difference was seen in breast cancer mortality during the prescreening period: 607 women died of breast cancer during 4.8 million person-years and 846 women died of breast cancer during 6.3 million person-years, respectively, in areas that did and did not adopt screening in the 40- to 49-year age group after 1986. From 1986 to 2005, 803 women during 7.3 million person-years and 1,238 women during 8.8 million person-years died of breast cancer in the intervention and control groups, respectively (estimated crude relative risk [RR], 0.79 [95% CI, 0.72 to 0.86]).

Women who were invited to screening had an RR of 0.74 (95% CI, 0.66 to 0.83), while those actually attending screening had an RR of 0.71 (95% CI, 0.62 to 0.80). Among women age 40 to 44, the RR was 0.83 (95% CI, 0.70 to 1.00) in those who were invited to screening and 0.82 (95% CI, 0.67 to 1.00) in those who attended; in women age 45 to 49, the corresponding RRs were 0.68 (95% CI, 0.59 to 0.78) and 0.63 (95% CI, 0.54 to 0.75), respectively. The authors estimated that 1,252 women (95% CI, 958 to 1,915) needed to be invited to screening over 10 years to save one life.

Undetected differences between the intervention and control groups might have affected the results, among other limitations, the authors noted. However, they concluded that screening mammography reduced death from breast cancer by 26% to 29% in women age 40 to 49 years. The benefit was greater in women age 45 to 49 than in those age 40 to 44, they said.