Purpose: Breast cancer is the most common cancer diagnosed among adolescent and young adult (AYA) females worldwide, but epidemiological patterns unique to this group are often obscured when results are combined with older patients. This study investigates breast cancer incidence and survival among AYA females, including differences by broad stage at diagnosis. Methods: A retrospective, population-based cohort study was conducted using de-identified data for females in Queensland, Australia, aged 15-39 diagnosed with a first primary breast cancer between 1997 and 2014 with follow-up to December 31, 2016. Incidence rate trends were examined with Joinpoint analysis. Cause-specific survival was calculated for key characteristics, and 5-year adjusted hazard ratios (HRs) were estimated from a multivariable flexible parametric model. Results: The study cohort comprised 2337 patients, of whom two-thirds (n = 1565, 67%) were diagnosed with advanced disease (tumor diameter >20 mm, lymph node involvement or presence of distant metastases at diagnosis). Incidence rates of localized tumors decreased by 1.9% per year (95% confidence interval [CI] -3.5% to -0.4%) over the study period, whereas the trend for advanced breast cancers remained stable. Five-year cause-specific survival increased from 85% to 92% for 2011-2014 compared to 1997-2001 (adjusted HR = 0.43, 95% CI = 0.29-0.65). Patients who were Indigenous from disadvantaged areas or diagnosed with advanced stage experienced significantly worse survival. Conclusion: The high proportion of younger females diagnosed with advanced breast cancer should be the focus of future campaigns to improve awareness and earlier detection. While survival has increased over time, further work is required to ensure that this progress is experienced equitably by all patients.
Keywords: Queensland; adolescents and young adults; breast cancer; incidence; survival.