Outcomes of patients treated with chemotherapy for breast cancer during pregnancy compared with nonpregnant breast cancer patients treated with systemic therapy.

Introduction: Prior studies of patients treated for breast cancer during pregnancy (PrBC) report mixed outcomes and are limited by substandard treatment, small cohorts, and short follow-up. This study compared survival outcomes of PrBC patients treated with chemotherapy during pregnancy with nonpregnant patients matched by age, year of diagnosis, stage, and subtype.

Methods: PrBC patients treated from 1989 to 2022 on prospective institutional protocols were eligible. Disease-free survival (DFS), overall survival (OS), and progression-free survival (PFS) were estimated using the Kaplan-Meier method and multivariable Cox proportional hazards regression.

Results: Among 143 PrBC and 285 nonpregnant patients, median follow-up was 11.4 years. Survival differences were statistically significant, with median DFS and OS not attained for PrBC patients versus 5.6 years (95% confidence interval [CI], 3.6-15.4; p = .0001) and 19.3 years (95% CI, 14.1-not estimated; p = .0262) for nonpregnant patients, respectively. Median PFS was 24.1 years (95% CI, 15.8-not estimated) for PrBC patients versus 8.4 years (95% CI, 6.4-10.9) for the nonpregnant cohort (p = .0008). Study cohort was associated with DFS, PFS, and OS in multivariable analyses, with the nonpregnant cohort having increased risks of disease recurrence (hazard ratio [HR], 1.91; 95% CI, 1.33-2.76; p = .0005) and disease progression or death (HR, 1.68; 95% CI, 1.19-2.39; p = .0035), and shorter OS (HR, 1.52; 95% CI, 1.01-2.29; p = .0442).

Conclusion: These data suggest that PrBC patients treated with chemotherapy during pregnancy have at least comparable, if not superior, outcomes than nonpregnant patients with similar age, cancer stage, and subtype. Analyses excluding patients with postpartum breast cancer were unable to be performed and are a priority for future confirmatory studies.