Background: We investigate rates of pathologic complete response (pCR) and tumor expression of ER, PgR, HER2 discordance after neoadjuvant chemotherapy using Japanese breast cancer registry data. Patients and methods: Records of more than 300 000 breast cancer cases treated at 800 hospitals from 2004 to 2013 were retrieved from the breast cancer registry. After data cleanup, we included 21 755 patients who received neoadjuvant chemotherapy and had no distant metastases. pCR was defined as no invasive tumor in the breast detected during surgery after neoadjuvant chemotherapy. HER2 overexpression was determined immunohistochemically and/or using fluorescence in situ hybridization. Results: pCR was achieved in 5.7% of luminal tumors (n ≥ 8730), 24.6% of HER2-positive tumors (n ≥ 4403), and 18.9% of triple-negative tumors (n ≥ 3660). Among HER2-positive tumors, pCR was achieved in 31.6% of ER-negative tumors (n ≥ 2252), 17.0% of ER-positive ones (n ≥ 2132), 31.4% of patients who received trastuzumab as neoadjuvant chemotherapy (n ≥ 2437), and 16.2% of patients who did not receive trastuzumab (n ≥ 1966). Of the 2811 patients who were HER2-positive before treatment, 601 (21.4%) had HER2-negative tumors after neoadjuvant chemotherapy, whereas 340 (3.4%) of the 9947 patients with HER2-negative tumors before treatment had HER2-positive tumors afterward. Of the 10 973 patients with ER-positive tumors before treatment, 499 (4.6%) had ER-negative tumors after neoadjuvant chemotherapy, whereas 519 (9.3%) of the 5607 patients who were ER-negative before treatment had ER-positive tumors afterward. Conclusion: We confirmed that loss of HER2-positive status can occur after neoadjuvant treatment in patients with primary HER2-positive breast cancer. We also confirmed that in practice, differences in pCR rates between breast cancer subtypes are the same as in clinical trials. Our data strongly support the need for retest ER, PgR, HER2 of surgical sample after neoadjuvant therapy in order to accurately determine appropriate use of targeted therapy.
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