TPS601 Background: The surgical treatment is a standard therapy for early breast cancer (EBC) after primary systemic therapy (PST). In more than half of HER2 positive (HER2(+)) breast cancer, pathological complete response (pCR) is achieved by PST with HER2 inhibitors and chemotherapy. In addition, hormone receptor (HR) negative HER2(+) (HR(-)HER2(+)) subtype has higher concordance between pCR and clinical complete response (cCR) before surgery than other subtypes, especially in EBC. However, non-surgical therapy is not an option for EBC with cCR after PST because of few evidence. We planned single arm confirmatory study to evaluate the efficacy and safety of the non-surgical therapy for HR(-)HER2(+) EBC with cCR after PST. Methods: The key eligibility criteria are as follows: 1) Histologically confirmed as invasive ductal carcinoma of breast, HR(-)HER2(+). 2) cT1-2, N0, M0 (UICC 8th). 3) No ipsilateral BC. 4) Women aged 20-74 years. 5) ECOG performance status 0 or 1. 6) Written informed consent. HER2 inhibitors (trastuzumab and pertuzumab) and cytotoxic drugs as PST are administered for all patients (pts). After completion of PST, cCR is diagnosed by breast imaging and physical examination. cCR is defined as 1) Not palpable breast mass by physical examination, 2) No enhanced breast mass by enhanced MRI, 3) No breast mass by sonography. After diagnosis of cCR, conventional radiotherapy for whole breast and boost radiation for tumor bed are mandatory, followed by pertuzumab and trastuzumab every 3 weeks during 9 months. In non-cCR cases, surgical resection is performed and adjuvant therapy are not specified. The primary endpoint is a distant metastasis-free survival (DMFS) at 3 year, the secondary endpoints are DFS, OS, RFS, proportion of local recurrence, and cosmetics outcome. Given that the threshold and expected of DMFS at 3-year is 93% and 98% with a significance level 2.5% (one sided) and 80% power, 170 cCR cases are required. Assuming half of HER2 pts reach to cCR, 350 pts are required as sample size started PST. Enrollment launched January, 2020 and 57 pts are enrolled as of January 12, 2021. Recent reports found that HR positive HER2(+) subtype has higher concordance between pCR and cCR by adding needle biopsy in the diagnosis, so we are planning to include HR positive subtype in this trial. This clinical trial has been registered at Japan Registry of Clinical Trials as jRCTs031190129 and conducted by the Japan Clinical Oncology Group (JCOG) Breast Cancer Study Group under public fund (National Cancer Center Research and Development Fund). Clinical trial information: jRCTs031190129.
Pathologic complete response to neoadjuvant anti-HER2 therapy is associated with HER2 immunohistochemistry score in HER2-positive early breast cancer
