Residual Cancer Burden (Rcb) As Prognostic In The I-Spy 2 Trial.

520Background: I-SPY2 is a multicenter phase 2 trial in high risk stage II/III breast cancer (BC) using adaptive randomization within biomarker subtypes to evaluate novel treatment agents added to standard neoadjuvant chemotherapy (NAC) in different phenotypic subsets of breast cancer. Residual cancer burden (RCB) quantifies the extent of residual disease (RD) for patients who did not achieve pathologic complete response (pCR = RCB-0). Methods: Local site pathologists reported RCB in the I-SPY2 trial. We performed a pooled analysis of 678 patients in I-SPY2 with RCB data and known follow-up (median 2.5 years). Cox models for event-free survival (EFS) were evaluated for RCB index (continuous) and RCB classes (hazard ratio; 95% CI) in all patients and in subtypes defined by hormone receptor (HR) and HER2 status. We separately compared experimental and control arms (Wilcoxon rank sum test) in a pooled analysis of RCB index (498 patients in total) from the first six treatment comparisons that “graduated” a th...