Updated overview of the Swedish Randomized Trials on Breast Cancer Screening with Mammography: age group 40-49 at randomization

The purpose of this overview is to estimate more precisely the long-term effect of mammography screening by adding four more years of follow-up to women aged 40-49 years in the four Swedish trials on mammography screening. Data from the four, trials were merged and linked to the Swedish Cancer and Cause of Death Register for 1958-1993 and 1951-1993 respectively to identify date of breast cancer diagnosis and cause and date of death. The invited and control groups comprised 48,569 and 40,247 women respectively. At the December 1993 follow-up, 602 and 482 breast cancer cases were identified in the two groups respectively, of which 104 and 111 had breast cancer as the underlying cause of death. This corresponds to a relative risk (RR) of 0.77 (95% CI: 0.59-1.01) for the two groups. In the 40-44 age group at randomization, 94% of breast cancer patients in the study and 89% in the control group were diagnosed before the age of 50; however, among breast cancer deaths in this age group, only two in the invited and five in the control group died after age 50. At follow-up of women 40-44 years at randomization 208 women in the invited and 184 in the control group were reported to the Cancer registry with breast cancer. Out of these 195 (94%) and 163 (89%) respectively were reported before the age of 50. Further, the relative risk for the age group 40-44 years at randomization by age at follow-up was 1.11, 0.51 and 0.46 for the age groups 45-49, 50-54, and 55-59 at follow-up. This study shows a 23% reduction in the breast cancer mortality in women 40-49 years at randomization achieved from a median trial time of 7.0 years, a median follow-up time of 12.8 years, and a screening interval of 18-24 months. Almost all of the effect in the 40-44 year age group at randomization was due to screening before the age of 50.