Benefit of adjuvant chemotherapy with or without trastuzumab in pT1ab node-negative human epidermal growth factor receptor 2-positive breast carcinomas: results of a national multi-institutional study

Benefit of adjuvant trastuzumab-based chemotherapy for node-positive and/or > 1 cm human epidermal growth factor receptor 2-positive (HER2+) breast carcinomas has been clearly demonstrated in randomized clinical trials. Yet, evidence that adjuvant chemotherapy with or without trastuzumab is effective in pT1abN0 HER2+ tumors is still limited. The primary objective of this study was to investigate the impact of adjuvant chemotherapy +/- trastuzumab on outcome in this subpopulation. A total of 356 cases of pT1abN0M0 HER2 + breast cancers were retrospectively identified from a large cohort of 22,334 patients, including 1248 HER2+ patients who underwent primary surgery at 17 French centers, between December 1994 and January 2014. The primary end point was disease-free survival (DFS). A multivariate Cox model was built, including adjuvant chemotherapy, tumor size, hormone receptor status, and Scarff Bloom Richardson (SBR) grade. A total of 138 cases (39%) were treated with trastuzumab-based chemotherapy, 29 (8%) with chemotherapy alone, and 189 (53%) received neither trastuzumab nor chemotherapy. Adjuvant chemotherapy +/- trastuzumab was associated with a significant DFS benefit (3-year 99 vs. 90%, and 5-year 96 vs. 84%, Hazard ratio, HR 0.26 [0.10-0.67]; p = 0.003, logrank test) which was maintained in multivariate analysis (HR 0.19 [0.07-0.52]; p = 0.001). Metastasis-free survival was also increased (HR 0.25 [0.07-0.86]; p = 0.018, logrank test) at 3-year (99 vs. 95%) and 5-year (98 vs. 89%) censoring. Exploratory subgroup analysis found DFS benefit to be significant in hormone receptor-negative, hormone receptor-positive, and pT1b tumors, but not in pT1a tumors. Adjuvant chemotherapy +/- trastuzumab is associated with a significantly reduced risk of recurrence in subcentimeter node-negative HER2+ breast cancers. Most of the benefit may be driven by pT1b tumors.