Post-9/11 deployment history and the incidence of breast cancer among women veterans.

PURPOSE:To determine if women Veterans who deployed in support of Operations Enduring Freedom/Iraqi Freedom (OEF/OIF) would show a greater likelihood of breast cancer (BC) than other women Veterans. METHODS:This was a retrospective cohort study of women aged <60 years who received Veterans Affairs medical center primary care, 2001-2021. The exposure was OEF/OIF deployment and the outcome was a BC diagnosis after entering Veterans Affairs care. Poisson models evaluated the association between deployment and BC incidence, covarying demographics, lifestyle factors, and hormonal contraceptive and hormone replacement therapy use. Analyses were also stratified by age and race, and a sensitivity analysis adjusted for healthcare utilization over the initial 2 years. RESULTS:Of 576,601 women, 24.6% (n = 141,935) deployed during post-9/11 conflicts. Across follow-up [median: 8.2 years], 1.2% women were diagnosed with BC. Those who deployed in support of OEF/OIF were 23% less likely to be diagnosed with BC than women who did not deploy (95% CI: 0.73, 0.86). The association remained in stratified models and when including healthcare utilization. CONCLUSIONS:Despite the exposures of OEF/OIF deployment, there was a significantly lower incidence of BC among women who deployed versus not, possibly due to a healthy soldier effect or to differences in screening.