Background: Recent studies indicate that the emergence of RAS/BRAF mutation sometimes occurs during initial anti-EGFR monoclonal antibody (mAb)-based treatment in metastatic colorectal cancer (mCRC). However, limited data was available regarding the timing and frequency of the emergence of RAS/BRAF mutation, and its impact on the efficacy of anti-EGFR mAb. With this background, we conducted an observational study to monitor RAS, BRAF, and PIK3CA mutation status by ctDNA in RAS/BRAF wt mCRC. Methods: RAS-trace is a pilot study to monitor RAS, BRAF, and PIK3CA mutation status by ctDNA, using NGS-based Plasma-Safe-SeqS technology for RAS/BRAF wt (diagnosed from tissue samples) mCRC treated with anti-EGFR mAb as a first-line chemotherapy. ctDNA- RAS/BRAF/PIK3CA status evaluated at baseline, 8, 12, 16, 20, 24, 36, 48 weeks and disease progression. The primary endpoint was the time to the acquired RAS mutations. The results of ctDNA at baseline and 8 weeks are presented. Results: Forty-two patients were enrolled, and 40 and 39 were used in the analysis of ctDNA at baseline and 8 weeks, respectively. All patients received mFOLFOX6 plus cetuximab (N=32) or panitumumab (N=8). At baseline, RAS, BRAF, and PIK3CA mutations were detected in 3, 3, and 1 of the 40 patients, respectively. One patient had both KRAS and BRAF mutations. Baseline ctDNA mutations were not associated with the presence of liver metastases, tumor sidedness, early tumor shrinkage, depth of response or skin rash ≥Grade 2. At 8 weeks, RAS, BRAF, and PIK3CA mutations were detected in 3, 2, and 0 among the 39 patients, respectively. One patient had both KRAS and BRAF mutations. ctDNA mutations at 8 weeks were not associated with the presence of liver metastases, tumor sidedness, or early tumor shrinkage, however, they were associated with depth of response and had a tendency of skin rashes ≥ Grade 2 (Table). Conclusions: Emergence of RAS/BRAF/PIK3CA mutations at 8 weeks may be related to the response of anti-EGFR mAb. The association of ctDNA status with survival outcomes will be presented in 2025. Clinical trial information: jRCT1050210160.