T1a triple negative breast cancer has the worst prognosis among all the small tumor (<1 cm) of TNBC and HER2-rich subtypes.

Background: Triple negative breast cancer (TNBC), accounting for 15% of all breast cancer cases, was usually considered as the most aggressive subtype. The present study evaluated the prognosis of T1a TNBC and the impact of tumor size on T1 TNBC survival in large-scale population.

Methods: This retrospective study enrolled T1a/T1b/T1c TNBC and HER2/hormone receptor (HoR) patients diagnosed between 2010 to 2012 from the Surveillance, Epidemiology, and End Results database. The following information was extracted for further analyses: demographic variables including age at diagnosis, race, marital status, laterality, histological grade, T/N stage, American Joint Committee on Cancer (AJCC) stage, radiation therapy, survival and cause of death. Kaplan-Meier method and Cox regression were engaged for breast cancer specific survival (BCSS) and overall survival (OS) analyses.

Results: In all, the present study enrolled 6,953 TNBC and 2,648 HER2/HoR patients. T1a TNBC which generally omitted adjuvant chemotherapy had worse prognosis than T1a HER2/HoR [BCSS: hazard ratio (HR) 3.23, 95% confidence interval (CI): 1.05-9.09, P=0.03; OS: HR 2.63, 95% CI: 1.25-5.56, P=0.01] and T1b HER2/HoR (BCSS: HR 5.26, 95% CI: 1.61-16.7, P=0.006; OS: HR 3.03, 95% CI: 1.27-7.14, P=0.013) tumors which both were recommended by the National Comprehensive Cancer Network (NCCN) guideline to have chemotherapy. T1a TNBC also showed a trend with poorer prognosis than T1b TNBC, but did not reach statistical significance.

Conclusions: T1a TNBC had the worst prognosis among all small tumors (<1 cm) of TNBC and HER2/HoR subtypes, indicating the necessity of more intensive adjuvant treatment.