Abstract P5-14-06: The effect of adjuvant chemotherapy in a large consecutive series of ER-pos HER-2 negative early breast cancers

Background: The last EBCTCG-overview reported, in general, a significant better breast cancer outcome if adjuvant chemotherapy was added to surgery and endocrine therapy in ER-pos breast cancers, but the precise indication for adjuvant chemotherapy in this population remains controversial. We study the effect of adjuvant chemotherapy on breast cancer outcome in consecutive patients with an ER-pos HER-2 negative breast cancer treated with adjuvant antihormonal therapy. Methods: Data were collected prospectively from consecutive patients with non-metastatic breast cancer that were primary operated between 2000 and 2012 at the University Hospitals Leuven (Belgium). A Propensity Score (PS) weighted analysis was performed to estimate the average treatment effect (ATE). The primary endpoint was recurrence free interval (RFI). Secondary endpoints were distant recurrence free interval (D-RFI) and breast cancer specific survival (BCSS). Covariates used to generate the propensity score and to include in the PS weighted analysis were age at diagnosis, body mass index, tumor size, grade, pN, lymph vessel invasion, PR, and radiotherapy. Cause-specific hazard models were fitted, using death not from breast cancer as competing risk. Treatment heterogeneity was examined by evaluating interactions of each covariate with adjuvant chemotherapy, using the Bonferroni-Holm method to correct for multiple testing. Results: In the total cohort of 5609 patients, 4282 had a hormone sensitive HER-2 neg breast cancer and 4121 (96.2%) of these received adjuvant antihormonal therapy. Adjuvant chemotherapy was given in 1179/4121 patients (29%). Median follow-up was 8.5 years. Due to very strong differences between patients with and without adjuvant chemotherapy, a restricted PS weighted analysis was used according to a recent recommendation in the statistical literature. This analysis is based on 1750 patients with a PS between 0.1 and 0.9. In this group, 807 (46%) received adjuvant chemotherapy, 211 (12%) observed an event for RFI, 167 (10%) for D-RFI, and 108 (6%) for BCSS. Adjuvant chemotherapy was associated with better prognosis: the adjusted cumulative incidence of recurrence within 5 years was 9.7% without and 5.3% with adjuvant chemotherapy. The adjusted hazard ratio for RFI was 0.50 (95% CI 0.33-0.74). There were no strong interactions with adjuvant chemotherapy. Results for D-RFI and BCSS were similar. Conclusion: Based on PS analysis to reduce confounding and chemotherapy indication bias, we observed clear benefit from adjuvant chemotherapy in ER-pos HER-2 negative breast cancers. Citation Format: Van Calster B, Neven P, Wildiers H, Punie K, Jongen L, Han S, Berteloot P, Van Nieuwenhuysen E, Nevelsteen I, Smeets A, Floris G. The effect of adjuvant chemotherapy in a large consecutive series of ER-pos HER-2 negative early breast cancers [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P5-14-06.