International comparisons of survival after recurrent metastatic breast cancer in four countries: A population-based study

Survival after metastatic breast cancer (MBC) has improved in high-income countries, yet international differences in outcomes and access to optimal care, particularly for recurrent disease remain unclear. We compared survival after recurrent MBC across four high-income countries, examining tumour subtype, treatment patterns and guideline adherence. Individual-level data were obtained from population-based cancer registries in Canada (British Columbia), Ireland, the Netherlands and the United States (Connecticut) for women who were diagnosed with stage I-III invasive breast cancer between 2005 and 2008 and developed distant metastatic recurrence between 2008 and 2010. Follow-up was from first recurrence until death, loss to follow-up or December 31, 2015. Kaplan-Meier methods were used to estimate overall survival, and age-standardised net survival (ASNS) at 1, 3 and 5 years after recurrence was estimated by registry and subtype. Among 2735 women with recurrent MBC, treatment at initial diagnosis varied across registries. Median survival after recurrence ranged from 12 months in Ireland to 18 months in the United States (p = 0.015). One-year ASNS ranged from 51.3% in Ireland to 63.6% in the United States and the Netherlands. Across countries, ASNS was highest for HR+/HER2- tumours and lowest for HR-/HER2- tumours. The Netherlands consistently showed the highest subtype-specific survival, while survival for HER2+ disease in Canada was closer to HR-/HER2- than HR+/HER2- disease. Differences narrowed over longer follow-up and in sensitivity analyses. Survival after recurrent MBC differed across these high-income countries. Improved harmonisation of recurrence data and timely implementation of evidence-based therapies may help reduce persistent international disparities.