Socio-economic inequalities in second primary cancer incidence: A competing risks analysis of women with breast cancer in England between 2000 and 2018.

We aimed to investigate socio-economic inequalities in second primary cancer (SPC) incidence among breast cancer survivors. Using Data from cancer registries in England, we included all women diagnosed with a first primary breast cancer (PBC) between 2000 and 2018 and aged between 18 and 99 years and followed them up from 6 months after the PBC diagnosis until a SPC event, death, or right censoring, whichever came first. We used flexible parametric survival models adjusting for age and year of PBC diagnosis, ethnicity, PBC tumour stage, comorbidity, and PBC treatments to model the cause-specific hazards of SPC incidence and death according to income deprivation, and then estimated standardised cumulative incidences of SPC by deprivation, taking death as the competing event. Multiple imputation was performed to account for missing data. Among 649,905 included women, 47,399 SPCs and 171,223 deaths occurred during 4,269,042 person-years of follow-up. Income deprivation was consistently associated with an increased rate of SPC incidence (cause-specific hazard ratio for the most vs. least deprived quintile: 1.29; 95% CI: 1.25, 1.33) and of death (1.36; 1.34, 1.38), translating into an absolute risk difference (the most vs. least deprived quintile) of 1.3% (95% CI: 1.0, 1.5) for SPC incidence and 4.9% (95% CI: 4.6, 5.1) for death at 10 years. Women with PBC from deprived areas in England faced a substantially higher risk of SPC than their counterparts. Future research is warranted to understand mechanisms for observed inequalities to inform strategies to monitor, prevent, and identify SPC in women with PBC.