Risks of second primary cancers among 584,965 female and male breast cancer survivors in England: a 25-year retrospective cohort study

Background Second primary cancers (SPCs) after breast cancer (BC) present an increasing public health burden, with little existing research on socio-demographic, tumour, and treatment effects. We addressed this in the largest BC survivor cohort to date, using a novel linkage of National Disease Registration Service datasets. Methods The cohort included 581,403 female and 3562 male BC survivors diagnosed between 1995 and 2019. We estimated standardized incidence ratios (SIRs) for combined and site-specific SPCs using incidences for England, overall and by age at BC and socioeconomic status. We estimated incidences and Kaplan–Meier cumulative risks stratified by age at BC, and assessed risk variation by socio-demographic, tumour, and treatment characteristics using Cox regression. Findings Both genders were at elevated contralateral breast (SIR: 2.02 (95% CI: 1.99–2.06) females; 55.4 (35.5–82.4) males) and non-breast (1.10 (1.09–1.11) females, 1.10 (1.00–1.20) males) SPC risks. Non-breast SPC risks were higher for females younger at BC diagnosis (SIR: 1.34 (1.31–1.38) <50 y, 1.07 (1.06–1.09) ≥50 y) and more socioeconomically deprived (SIR: 1.00 (0.98–1.02) least deprived quintile, 1.34 (1.30–1.37) most). Interpretation Enhanced SPC surveillance may benefit BC survivors, although specific recommendations require more detailed multifactorial risk and cost-benefit analyses. The associations between deprivation and SPC risks could provide clinical management insights. Funding CRUK Catalyst Award CanGene-CanVar (C61296/A27223). Cancer Research UK grant: PPRPGM-Nov 20∖100,002. This work was supported by core funding from the NIHR Cambridge Biomedical Research Centre (NIHR203312)]. The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care.