Abstract A57: Evaluation of the pathologic predictors of treatment response to neoadjuvant chemotherapy for invasive lobular breast cancer

Introduction: Neoadjuvant chemotherapy (NAC) is becoming the standard of care for locally advanced breast cancer as a tool to objectively measure treatment response prior to surgery, and to downstage tumors to permit breast conserving surgery. Invasive lobular carcinoma (ILC) comprises approximately 10% of all invasive breast cancers, but is less likely than invasive ductal carcinoma to respond to NAC. The purpose of this study was to explore pathological and molecular predictors of response to NAC in patients with ILC. Study Methods: Retrospective pathology review was conducted of all consecutive cases of non-metastatic ILC patients who had received NAC and subsequent surgery for ILC (n=32) within the Allina Health system. Pre-treatment tumor size, grade, stage, HER2 status, hormone receptor status, sub-type, Ki-67 index, and post-treatment Residual Cancer Burden (RCB) were determined by breast pathologists. Mann-Whitney U and Chi-square tests with exact methods were used to test for between group difference of responders (RCB 0-1) versus non-responders (RCB 2-3) to NAC on continuous and dichotomous variables. Multivariate logistic regression was conducted to determine significant predictors of response to NAC. Findings: On univariate analysis, pleomorphic subtype (p=.004), Ki-67 (p=0.013), and HER2 status (p=.054) were significant predictors of response to NAC. On multivariate analysis, HER2 and Ki-67 provided the most efficient model for predicting response to NAC. This model correctly classified 81.3% of patients, 81.8% of RCB 0/1 and 80% of RCB 2/3. Pleomorphic subtype was highly correlated to KI-67. Conclusions: Despite a limited sample size, this study shows that pathologic predictors (pleomorphic subtype, Ki-67, and HER2 status) are significant predictors of response to NAC, and may be useful in determining which ILC patients will benefit from NAC. This is an important finding because NAC for non-responders may delay the initiation of surgery and hormonal treatments for ILC. Citation Format: Michaela L. Tsai, Marsha J. Finkelstein, Tamera J. Lillemoe, Barbara Susnik, Erin Grimm, Sung-Hae Kang, Caitlin Kelly, Karen K. Swenson. Evaluation of the pathologic predictors of treatment response to neoadjuvant chemotherapy for invasive lobular breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A57.