Adjuvant therapy for small, node-negative HER2-positive breast cancer (HER2 BC) is controversial. We aimed to identify the subgroup that would benefit most from adjuvant chemotherapy and trastuzumab.

We reviewed records of patients with pT1N0M0 HER2 BC treated at our institution from January 1, 1998, through October 31, 2009. We compared three groups: A, no adjuvant chemotherapy; B, adjuvant chemotherapy only; and C, adjuvant chemotherapy with trastuzumab. We evaluated disease-free survival (DFS), overall survival (OS), distant recurrence-free survival (DRFS), and breast cancer-specific survival (BCSS) in each group.

We reviewed 587 consecutive patients with a median follow-up of 123.0 months. The 10-year DFS rate was 81.0%, 65.4%, and 97.3% in groups A, B, and C, respectively ( < 0.001). The restricted mean survival time ratio did not differ between groups A and B [ratio = 0.982; 95% confidence interval (CI), 0.930-1.036; = 0.498). Cox regression showed that adjuvant chemotherapy with trastuzumab was associated with better DFS compared with no adjuvant chemotherapy [hazard ratio (HR), 0.071; 95% CI, 0.025-0.204; < 0.001). Larger tumor size was associated with short DFS (HR, 2.384; 95% CI, 1.549-3.056; < 0.001); improvements in DFS, OS, DRFS, and BCSS were observed with adjuvant chemotherapy plus trastuzumab in patients with tumors ≥0.8-cm diameter. Receiving adjuvant chemotherapy with trastuzumab was not associated with improved DFS, OS, or DRFS for tumors <0.8 cm.

Adjuvant chemotherapy plus trastuzumab should be recommended for patients with pT1N0M0 HER2 BC ≥0.8 cm in diameter; adjuvant therapy may not be necessary for tumors <0.8 cm.