Adjuvant chemotherapy and survival among patients 70 years of age and younger with node-negative breast cancer and the 21-gene recurrence score of 26–30

BACKGROUND: The benefits of chemotherapy in node-negative, hormone receptor-positive, and human epidermal growth factor receptor 2 (HER2)-negative breast cancer patients with the 21-gene recurrence score (RS) of 18–30, particularly those with RS 26–30, are not known. METHODS: Using the Surveillance, Epidemiology, and End Results (SEER) data, we retrospectively identified 29,137 breast cancer patients with the 21-gene RS of 18–30 diagnosed between 2004 and 2015. Mortality risks according to the RS and chemotherapy use were compared by the Kaplan-Meier method and Cox’s proportional hazards model. RESULTS: Among the breast cancer patients with the RS 18–30, 21% of them had RS 26–30. Compared to breast cancer patients with RS 18–25, patients with RS 26–30 had more aggressive tumor characteristics and chemotherapy use and increased risk of breast cancer-specific mortality and overall mortality. In breast cancer patients who were aged ≤ 70 years and had RS of 26–30, chemotherapy administration was associated with a 32% lower risk of breast cancer-specific mortality (hazard ratio [HR], 0.68; 95% confidence interval [CI], 0.47–0.99) and a 42% lower risk of overall mortality (HR, 0.58; 95% CI, 0.44–0.76). Survival benefits were most pronounced in breast cancer patients who were younger or had grade III tumor. CONCLUSIONS: The 21-gene RS of 18–30 showed heterogeneous outcomes, and the RS 26–30 was a significant prognostic factor for an increased risk of mortality. Adjuvant chemotherapy could improve the survival of node-negative, hormone receptor-positive, and HER2-negative breast cancer patients with the 21-gene RS 26–30 and should be considered for patients, especially younger patients or patients with high-grade tumors. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13058-019-1190-4) contains supplementary material, which is available to authorized users.