Background: Image-guided vacuum-assisted biopsy (VAB) is increasingly used after completion of neoadjuvant chemotherapy (NAC) to assess residual disease in the breast, facilitate risk-adaptive surgery and potentially identify exceptional responders who may not require surgical intervention. The aim of this analysis was to investigate the diagnostic performance of a standardized post-NAC VAB protocol, developed following retrospective analysis of institutional data (1). Methods: Prospective cohort study of patients with HER2 positive and triple negative (TN) invasive ductal carcinoma, treated with NAC, who had partial/complete imaging response and underwent post-NAC VAB to aid surgical planning between 02/2018 and 06/2019. The aim of VAB was to sample the site of residual imaging abnormality (breast residuum <2cm) previously marked by clip insertion. Pathologic complete response (pCR) was defined as no residual disease in the breast (ypT0). Diagnostic accuracy of VAB was calculated using final surgical pathology as the reference standard. Simple descriptive statistics were performed. Results: 26 eligible patients underwent post-NAC VAB. This was representative in 23 cases. The overall pCR rate was 46.2% (42.1% for HER2 positive, 57.1% for TN phenotypes). The post-NAC VAB false negative rate (FNR) was 9.1% (95% CI: 0-26.1) and the negative predictive value (NPV) was 90.91% (95% CI: 60.27-98.51) with an overall accuracy of 86.96% (95% CI: 66.41-97.22). Conclusions: This data suggests that post-NAC VAB may reliably predict pCR in patients with HER2 positive and TN invasive ductal carcinoma with good response to NAC. Further technical refinements in VAB technique, standardization in patient selection and prospective trials are warranted to further explore the role of post-NAC VAB in supporting minimal or no surgery trials. References 1. Tasoulis MK, Roche N, Rusby JE, Pope R, Allen S, Downey K, Nerurkar A, Osin P, Wilson R, MacNeill F. Post neoadjuvant chemotherapy vacuum assisted biopsy in breast cancer: Can it determine pathologic complete response before surgery? J Clin Oncol 2018;36 (Supplement): abstr 567.