Clinical characteristics and disease outcomes in ER+ breast cancer: a comparison between HER2+ patients treated with trastuzumab and HER2- patients

Trastuzumab has changed the prognosis of HER2+ breast cancer. We aimed to investigate the prognosis of ER+/HER2+ patients treated with trastuzumab, thus to guide escalation endocrine treatment in ER+ breast cancer. ER-positive early breast cancer patients operated at Ruijin Hospital between Jan. 2009 and Dec. 2017 were retrospectively included. Eligible patients were grouped as HER2-negative (HER2-neg) or HER2-positive with trastuzumab treatment (HER2-pos-T). Kaplan-Meier analysis and Cox proportional hazards model were used to compare the disease-free survival (DFS) and overall survival (OS) between these two groups. A total of 3761 patients were enrolled: 3313 in the HER2-neg group and 448 in the HER2-pos-T group. Patients in the HER2-pos-T group were associated with pre/peri-menopause, higher histological grade, LVI, higher Ki-67 level, lower ER and PR levels (all P <  0.05). At a median follow-up of 62 months, 443 DFS events and 191 deaths were observed. The estimated 5-year DFS rate was 89.7% in the HER2-neg group and 90.2% in the HER2-pos-T group (P = 0.185), respectively. Multivariable analysis demonstrated that patients in the HER2-pos-T group had a better DFS than patients in the HER2-neg group (HR 0.52, 95% CI: 0.37–0.73, P <  0.001). The estimated 5-year OS rates were 96.0% and 96.3% in the two groups, respectively (P = 0.133). Multivariate analysis found that HER2-pos-T group was still associated with significantly better OS compared with the HER2-neg group (HR 0.38, 95% CI: 0.22–0.67, P = 0.037). ER+/HER2+ breast cancer patients treated with trastuzumab were associated with superior outcome compared with ER+/HER2- patients, indicating HER2-positivity itself may not be an adverse factor for ER+ patients in the era of trastuzumab.