Eva Claassens, Maastricht University

Background:
Breast cancer patients with residual disease in the breast and/or axilla after neoadjuvant systemic therapy (NST) can benefit from adjuvant systemic therapy (AST). Trastuzumab and emtansine (T-DM1) in HER2+ and capecitabine in triple-negative (TN) breast cancer improve disease-free and overall survival. We aimed to investigate how often the indication for AST in HER2+ and TN breast cancer is based on residual disease in the axilla rather than residual disease in the breast. Additionally, we assessed whether the use of less invasive axillary staging procedures may affect decision-making regarding AST.

Methods:
All clinically node-positive (cN+) breast cancer patients treated with NST in the RISAS trial (a prospective study on the diagnostic accuracy of Targeted Axillary Dissection [TAD]) were analyzed. Patients were categorized based on pathologic complete response (pCR) of the breast and/or axilla. Next, in patients with breast-only pCR, it was assessed how often sentinel lymph node biopsy (SLNB), MARI and RISAS (compared to axillary lymph node dissection (ALND)), resulted in missing the indication for AST if no ALND would have been performed.

Results:
In a total of 210 cN+ patients, 50 patients (23.8%) had a pCR of the breast and axilla and 160 (76.2%) had residual disease. In 109 patients with HER2+ (n=64) and TN (n=45) breast cancer, 63 (57.8%) had residual disease in the breast and/or axilla and thus an indication for AST. This indication was based solely on axillary residual disease in 10/63 (15.9%) patients. Regarding the risk of missing the AST indication, RISAS and MARI had 3.2%(2/63) risk while SLNB had 4.8%(3/63) risk.

Conclusion:
In a considerable number of cN+ patients with HER2+ and TN breast cancer, the indication for AST is based on residual disease in the axilla. An axillary staging procedure with a low risk of missing residual disease, such as TAD, is important for appropriately guiding AST.