Impact of less invasive axillary staging procedures after neoadjuvant systemic therapy on adjuvant systemic therapy indications in HER2-positive and triple-negative breast cancer.

[OBJECTIVE] To determine how often the indication for adjuvant systemic therapy (AST) in HER2+ and TN breast cancer is based solely on residual axillary disease (ypT0N+) and to estimate the theoretical frequency with which less invasive axillary staging procedures may fail to identify AST eligibility.

[BACKGROUND] HER2+ and triple-negative (TN) breast cancer patients with residual disease after neoadjuvant systemic therapy (NST) benefit from AST with trastuzumab and emtansine (T-DM1) and capecitabine improving disease-free and overall survival for HER2+ and TN patients, respectively.

[METHOD] This retrospective analysis of prospectively collected data from the multicenter RISAS trial included clinically node-positive (cN+) breast cancer patients treated with NST. We assessed how often ypT0N + occurred and estimated how often sentinel lymph node biopsy (SLNB), marking axillary lymph nodes with radioactive iodine seeds (MARI) or RISAS might theoretically fail to detect residual axillary disease, compared to axillary lymph node dissection (ALND).

[RESULTS] In 109 HER2+ (n = 64) and TN (n = 45) breast cancer patients, 63 (57.8%) had residual disease in the breast and/or axilla and were eligible for AST. Eligibility was based on ypT0N+ in 10/63 (15.9%) patients. The theoretical risk of missing AST eligibility was 3.2% (2/63) for RISAS, and 4.8% (3/63) for MARI and SLNB.

[CONCLUSIONS] In this cN + cohort, approximately one in six patients eligible for AST had residual disease only in the axilla. Less-invasive axillary staging procedures were associated with a low estimated theoretical risk of missed AST eligibility. However, these findings should be interpreted in light of the modest sample size.