The prognosis of patients with stage II-III Human Epidermal growth factor Receptor 2 (HER2)-positive breast cancer has significantly improved since the addition of trastuzumab to (neo-)adjuvant chemotherapy. Several reports have shown that small (≤2 cm), node-negative, HER2-positive tumors have a relatively poor prognosis and these patients increasingly receive trastuzumab-based chemotherapy. We aimed to provide evidence for this approach in a population-based cohort. All T1N0M0 HER2-positive breast cancer patients diagnosed between 2006 and 2012 were identified from the Netherlands Cancer Registry. Patient, tumor, and treatment characteristics were recorded. Kaplan-Meier statistics were used for overall survival (OS) and breast cancer-specific survival (BCSS) estimations overall and in T1a, T1b, and T1c tumors separately. Cox regression analyses were performed to account for imbalances in baseline characteristics between treated and untreated patients. A total of 3512 patients were identified: 385 with T1a, 800 with T1b, and 2327 with T1c tumors. Forty-five percent of patients received chemotherapy and/or trastuzumab: 92 % received both. Chemotherapy and/or trastuzumab significantly improved 8-year OS (95 vs. 84 %; hazard ratio [HR] 0.29; 95 % confidence interval [CI] 0.21-0.41, P < 0.001). The effect remained significant in multivariable analyses (HR 0.35; 95 % CI 0.23-0.52, P < 0.001). BCSS was also improved with systemic treatment in univariable (96 vs. 92 %; HR 0.41; 95 % CI 0.27-0.63, P < 0.001) and multivariable analyses (HR 0.31; 95 % CI 0.19-0.53, P < 0.001). Treatment effect on OS and BCSS was similar in T1a, T1b, and T1c tumors. Chemotherapy and/or trastuzumab improves OS and BCSS and can be considered in all patients with small node-negative HER2-positive breast cancer.
Keywords: Breast cancer; HER2; Overall survival; Systemic treatment; Trastuzumab.