Influence of Mammographic Screening on Breast Cancer Incidence Trends in South Australia

Purpose: To examine breast cancer (BC) incidence trends in relation to mammographic screening and riskfactor prevalence in South Australia (SA). Materials and
Methods:Trends in annual BC incidence rates werecalculated using direct standardisation and compared with projected incidence derived from Poisson regressionanalysis of pre-screening rates. Annual percentage change and change time points were estimated using Joinpointsoftware. Biennial mammography screening participation rates were calculated using data from BreastScreenSA. Trends in overweight/obesity, alcohol use and hormone replacement therapy (HRT) use were examined using1991-2009 Health Omnibus Survey data. Trends in total fertility were examined using data from the AustralianBureau of Statistics.
Results: BC incidence increased around the time BreastScreen commenced and thenstabilised in the mid-1990s. However rates have remained higher than projected, even though the proportionand age distribution of first time screening attendees stabilised around 1998. A decrease in BC incidence wasobserved among women aged 50-59yrs from the late-1990’s but not among older women. Obesity and alcoholuse have increased steadily in all age groups, while HRT use declined sharply from the late-1990s.
Conclusions:BC incidence has remained higher than projected since mammography screening began. The sustained elevationis likely to be due to lead time effects, though over-diagnosis cannot be excluded. Declining HRT use has alsoimpacted incidence trends. Implications: Studies using individual level data, which can account for changes inrisk factor prevalence and lead time effects, are required to evaluate ‘over-diagnosis’ due to screening.