optimization of adjuvant chemotherapy, and early detection of recurrence. We present here the study design of an ongoing trial.Trial design: GALAXY (UMIN000039205) is a prospective observational study to monitor the ctDNA status and to establish the registry data in patients with resectable CRC who underwent radical surgery. The study utilizes a personalized, tumor-informed ctDNA assay that is designed to track 16 patient-specific somatic variants (SignateraÔ bespoke multiplex-PCR NGS assay) based on whole-exome sequencing of tumor tissue sample. Blood samples will be collected at the following time points: at pre-surgery and 1, 3, 6, 9, 12, 18, and 24 months post-surgery, and at the same time the computed tomography (CT) image will be performed. Mutations in RAS, BRAF and microsatellite instability tests by validated PCR methods will be assessed centrally. Key eligibility criteria include histologically confirmed colorectal adenocarcinoma and clinical stage IIeIV or relapsed disease amenable to radical surgical resection. The primary endpoint is disease-free survival. Key secondary endpoints are overall survival, ctDNA status at each time point, association between clinical characteristics and gene alterations, and association between ctDNA detection and relapse detection by the CT image (that is, whether ctDNA monitoring could replace the CT image). There is no pre-planned statistical hypothesis due to the nature of observational study. Target sample size is 2,500 patients. Based on ctDNA results in the GALAXY trial, patients can be enrolled in investigator-initiated phase 3 trials, either the VEGA (deescalation) or the ALTAIL trial (escalation). The VEGA trial assesses the non-inferiority of observation vs. adjuvant CAPOX in GALAXY participants with an absence of ctDNA 1 month post-surgery. The ALTAIR trial evaluates the superiority of FTD/TPI over placebo in GALAXY participants with ctDNA status that remains positive after standard therapy. Moreover, additional tumor tissue and blood samples will be banked for further translational research.