Impact Of Metaplastic Histology (Mpbc) In Triple-Negative Breast Cancer (Tnbc) Patients (Pts) Receiving Neoadjuvant Systemic Therapy (Nast)

593Background: MpBCs are aggressive cancers, often TNBC, and considered chemo-resistant, such that some providers avoid NAST. Using data from an IRB approved prospective trial, we compared characteristics and outcomes between MpBC and non-metaplastic (non-MpBC) TNBC pts receiving NAST. Methods: Per protocol, pts began a planned 4 cycles of Adriamycin-based chemo (AC). Volumetric change by ultrasound (vUS) was performed after 2 (optional) and 4 cycles of AC and evaluated as a predictor of response in MpBC pts. Pts with chemo-insensitive disease during/after AC were enrolled in targeted therapy trials (TT) as the 2nd phase of NAST. Sensitive disease received taxane-based therapy (T). Residual Cancer Burden (RCB) was assessed after surgery. Differences between MpBC and non-MpBC were analyzed by Fisher’s exact, Wilcoxon rank sum and Student’s t tests as appropriate. Results: 170 pts (21 MpBC, 149 non-MpBC) were evaluable for RCB including 2 pt deaths during AC (counted as RCB II-III). Despite lower rates of n...