Co-expressed type of ER and HER2 protein as a predictive factor in determining resistance to antiestrogen therapy in patients with ER-positive and HER2-positive breast cancer

900 Background: The purpose of this study is to analyze whether or not inter-site variation types on ER and HER2 expression may be a predictive factor for evaluating effectiveness of endocrine therapy in patients with ER-positive and HER2-positive breast cancer. Methods: A total of 366 consecutive women with invasive breast cancer who had undergone curative surgical treatment between 1996 and 2001 were included in this study. ER status was evaluated using the Allred score and HER2 according to the HercepTest. In ER-positive and HER2-positive tumors, expression of ER and HER2 was described as the co-expressed type or the differently expressed type using double staining with ER and HER2 antibody. Results: Of the 366 patients, 249 (68.1%) were positive for ER and 74 (20.2%) were positive for HER2. ER-positive and HER2-negative tumors were found in 221 patients (60.4%), ER-negative and HER2-negative in 71 (19.4%), ER-negative and HER2-positive in 46 (12.6%), and ER-positive and HER2-positive in 28 (7.7%). HER...