Background: Achieving pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) is known important for predicting survival. However, few studies have reported prognosis of minimal residual disease. In this study, we evaluated the long term survival of the patients with breast or nodal pCR and compare patients with minimal residual disease to the breast and node pCR group. As it is difficult to analyze residual cancer burden and tumor-infiltrating lymphocytes in the retrospective study, we assessed the residual tumor or nodal status according to the pathologic report after surgery based on the AJCC 8th edition staging system. Methods: This study retrospectively reviewed 753 patients who achieved pCR of breast or node after NAC from 2010 to 2017 at Severance Hospital. We divided patients into five groups based on the residual disease; breast and node pCR (ypT0/TisN0), minimal residual disease (ypT1mi/T1aN0, ypT0/TisN0i+/N1+), breast-only pCR (ypT0/TisN1+), node-only pCR with residual tumor size no larger than 20mm (ypT1b/T1cN0), and node-only pCR with residual tumor size larger than 20mm (ypT2+N0). The basic characteristics and the 10-year event free survival (EFS) were evaluated. EFS was defined as time from surgery to last follow-up date without any events. Results: We analyzed the data until May 26, 2021, and 50.2% (n=378) of the patients achieved breast and node pCR. Minimal residual disease, defined as residual tumor size no larger than 5mm and residual nodal size 2mm, included 126 patients (16.7%). Overall, there was significant difference in 10-year EFS among five groups (ypT0/TisN0; 94.7%, ypT1mi/T1aN0, ypT0/TisN0i+/N1+; 93.7%, ypT1b/T1cN0; 86.8%, ypT0/TisN1+; 87.9%, ypT2+N0; 71.9%, p<0.001). As the variables such as age, subtypes, clinical T and N stage, regimen of chemotherapy, response to the chemotherapy and operation type were adjusted, the minimal residual disease group showed no significant difference with the breast and node pCR group (HR 1.410, 95% CI 0.615 - 3.235, p=0.417), yet the other groups showed poorer outcomes (ypT1b/T1cN0; HR 3.183, 95% CI 1.687- 6.004, p<0.001, ypT0/TisN1+; HR 3.186, 95% CI 1.047 - 9.694, p=0.041, ypT2+N0; HR 6.089, 95% CI 3.019- 12.281, p<0.001). Conclusion: Patients with minimal residual tumor 5mm or less, or residual metastatic lymph node with a maximal diameter of 2mm have shown equivalent 10-year EFS compared to the patients who achieved breast or node pCR after NAC. Keywords: Neoadjuvant chemotherapy, Pathologic complete response, Minimal residual disease, Long-term survival, Breast cancer

Citation Format: Jieon Go, Jee Hyun Ahn, Jung Min Park, Soon Bo Choi, Jee Ye Kim, Hyung Seok Park, Seung Il Kim, Byeong-Woo Park, Seho Park. Long-term prognosis of minimal residual disease in breast cancer patients with breast or nodal pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-07-14.