A US Registry-Based Assessment of Use and Impact of Chemotherapy in Stage I HER2-Positive Breast Cancer.

Background: Despite the paucity of evidence supporting chemotherapy in the treatment of node-negative, HER2-positive breast cancer measuring <2 cm, use of trastuzumab-based chemotherapy has increased over the past decade. Therefore, we used the National Cancer Database to evaluate the use and impact of chemotherapy on survival in this population. Methods: We identified female patients aged 18 to 70 years with node-negative, HER2-positive breast cancer measuring <2 cm. A propensity-matched cohort model was used to control for risk factors known to influence survival. Primary end points assessed were receipt of chemotherapy and overall survival (OS). Results: In our propensity-matched cohort model (n=8,222), adjuvant chemotherapy (ACT) was associated with a lower 5-year OS rate in Tlmi breast cancer (n=626; 89.1% [95% CI, 81.8%-93.5%] vs 99.1% [96.6%-99.8%]), no significant effect in T1a disease (n=2,901; 95.4% [93.2%-96.9%] vs 96.9% [94.1%-98.3%]), and improved 5-year OS in Tlb (n=2,340; 97.1% [95.1%-98.4%) vs 92.3% [88.5%-94.9%)) and T1c tumors (n=2,355; 95.9% [93.5%-97.5%) vs 91.5% [88.4%-93.9%]). In the entire cohort of 21,148 patients who met the inclusion criteria, ACT was associated with lower 5-year OS in T1mi (89.6% [83.7%-93.4%] vs 98.1% [96.6%-98.9%]) and T1a tumors (94.9% [92.9%-96.3%] vs 96.5% [94.6%97.7%]), and improved 5-year OS in T1b (96.8% [95.6%-97.7%) vs 92.3% [88.7%-94.8%]) and T1c tumors (95.8% [94.9%-96.5%] vs 91.6% [88.5%-93.9%]). Increased use of ACT was observed over the study period. From 2010 to 2013, annual treatment rates were 71.5%, 72.4%, 73.3%, and 74.4%, respectively (trend test, P<.0001). Condusions: Our data support the use of ACT for HER2-positive, node-negative T1b and Tic breast cancer, whereas no benefit was observed for ACT in T1mi and T1a HER2-positive, node-negative breast cancer. Although use of ACT is increasing in node-negative, HER2-positive breast cancer <2 cm, our findings caution against its use in the smallest of these tumors (T1mi and T1 a) due to lack of survival benefit.