Population screening and intensity of screening are associated with reduced breast cancer mortality: evidence of efficacy of mammography screening in Australia

Abstract

Efficacy of breast screening may differ in practice from the results of randomized trials. We report one of the largest case–control evaluations of a screening service. Subjects included 491 breast-cancer deaths affecting 45–80-year-old South Australian females during 2002–2005 (diagnosed after BreastScreen commencement) and 1,473 live controls (three per death) randomly selected from the State Electoral Roll after birth-date matching. Cancer Registry and BreastScreen records provided cancer and screening details. Risk estimates were calculated by BreastScreen participation, using conditional logistic regression. Interpretation was assisted by a population survey of risk factor prevalence by BreastScreen participation in 1,684 females aged ≥40 years. The relative odds (OR) (95% confidence limits) of breast-cancer death in BreastScreen participants compared with non-participants were 0.59 (0.47, 0.74). Compared with non-participants, the OR was 0.70 (0.47, 1.05) for women last screened through BreastScreen more than 3 years before diagnosis of the index case, and 0.57 (0.44, 0.72) for women screened more recently. The OR of 0.47 (0.34, 0.65) for women screened more frequently in the pre-diagnosis phase was lower than the 0.64 (0.50, 0.82) for other screened women. The overall OR of 0.59 approximated 0.70 when corrected for the screening self-selection bias observed in five randomized trials. However, multivariable analysis of survey data did not indicate a lower prevalence of breast-cancer risk factors among BreastScreen participants, suggesting that this correction may be inappropriate. Participation in screening was associated with a breast-cancer mortality reduction of between 30 and 41%, depending on assumptions about screening self-selection bias. A downward mortality risk by recency of last screen prior to cancer diagnosis, and frequency of recent screening, is consistent with a screening effect.