Earlier screening, better outcomes? Revisiting breast cancer screening guidelines for women in their 40s.

While mammography screening programmes improve early detection and reduce mortality for individuals aged 50-74, its extension to those aged 40-49 remains debated. In Canada, breast screening eligibility varies between provinces/territories, with Ontario lowering its eligibility age from 50 to 40 in 2024. This commentary examines recent evidence, including observational studies and simulation models, suggesting that mammography screening from age 40 may offer net benefits. Additionally, using data from Ontario Health (Cancer Care Ontario), we compared stage at diagnosis and 5-year survival rates among 18,639 women aged 40-51 diagnosed with breast cancer (2009-2017). Individuals aged 40-49 had comparable stage at diagnosis and 5-year survival rates to unscreened individuals aged 50-51. Meanwhile, screened individuals aged 50-51 demonstrated the earliest stage at diagnosis and highest 5-year survival rate. Our analysis illustrates the arbitrary nature of an age-based screening threshold at 50. We demonstrate that outcomes for women aged 40-49 resemble those of unscreened women aged 50-51, who were just above the eligibility cutoff. While expanding screening may increase upfront costs, these could be offset by avoiding late-stage treatments and integrating risk-stratified approaches. Overall, women in their 40s may benefit from organized screening programs through earlier detection and improved survival.