Clinical outcomes of early-stage triple-negative breast cancer after neoadjuvant chemotherapy according to HER2-low status

The impact of human epidermal growth factor receptor 2 (HER2) expression determined by immunohistochemistry (IHC) on outcomes in early-stage triple-negative breast cancer (eTNBC) is unclear. Using a large, multi-institutional cohort, we evaluated outcomes by HER2 IHC status in patients with eTNBC who received neoadjuvant therapy (NAT). Patients with stage I-III TNBC who received NAT and underwent surgery from January 2016 to June 2019 were identified from three databases. HER2 expression was defined as low (IHC1+ or 2+/FISH not amplified) or HER2 IHC score 0 by local testing at diagnosis. Pathological complete response (pCR) rates were compared using logistic regression adjusted for multiple factors. Survival outcomes were estimated using Kaplan–Meier and Cox proportional hazards models. Among 977 consecutive patients, 388 (39.7%) had HER2-low and 589 (60.3%) had HER2 IHC score 0 tumors. Median age at eTNBC diagnosis was 50.3 years (range 21.0-83.4 years). At baseline, clinical nodal positivity rate was significantly higher in HER2-low (55.0%) versus HER2 IHC score 0 tumors (46.6%) (P = 0.011); pCR rates were similar (32.0% versus 32.6%; adjusted P = 0.924). At a median follow-up of 3.5 years, recurrence-free survival (RFS) did not vary significantly between HER2-low versus HER2 IHC score 0 among patients with pCR (adjusted P = 0.368) or residual disease (RD) after NAT (adjusted P = 0.573). Distant RFS and overall survival (OS) did not differ by HER2 category for patients with pCR [distant RFS (DRFS), adjusted P = 0.509; OS, adjusted P = 0.514] or RD (DRFS, adjusted P = 0.812; OS, P = 0.285). Discordance of tumor HER2 status was seen in 31.1% of HER2 IHC score 0 cases, with HER2 expression observed post-treatment; 34.8% of HER2-low cases showed discordance, with absent HER2 expression in RD. In this large cohort of patients with eTNBC treated with NAT, HER2-low status was not associated with pCR or survival after adjusting for clinical factors. The discordance in HER2 IHC pre- and post-NAT likely reflects challenges in HER2 quantification and heterogeneity. •HER2-low eTNBC showed no pCR or survival benefit over HER2 IHC score 0.•RFS did not significantly differ between HER2 IHC categories after NAT.•HER2-low tumors had higher nodal positivity (55%) than HER2 IHC score 0 (46.6%); P = 0.011.•Post-treatment HER2 IHC status changed in 31.1% of HER2 IHC score 0 and 34.8% of HER2-low cases.•HER2 IHC status discordance underscores quantification and heterogeneity