Díaz-Rubio, E E

e15557 Background: In metastatic colorectal cancer (mCRC), systemic chemotherapy can enable conversion surgery in a subset of patients with initially unresectable disease. Although R0 resection considered a key determinant of long-term survival, its clinical impact across heterogeneous trial populations and treatment strategies remains insufficiently defined. We aimed to clarify the survival benefit of R0 resection after first-line chemotherapy using large-scale individual patient data. Methods: Using the ARCAD database, which comprises 67 randomized clinical trials and approximately 47,000 patients, we conducted a pooled analysis of individual patient data from six first-line randomized clinical trials (COIN, COIN-B, 03-TTD-01, ATOM, SOFT and PARADIGM) for which data on R0 resection were available. Patients with initially unresectable mCRC and available surgical data were categorized into three groups: R0 resection, non-R0 resection, and no surgery. All R0 cases were included, comprising liver-only, lung-only, combined liver and lung metastases, and peritoneal dissemination or other metastatic sites. Overall survival (OS) and progression-free survival (PFS) were evaluated using multivariable Cox proportional hazards models adjusted for age, sex, and performance status. Pre-specified subgroup analyses examined the consistency of outcomes according to tumor response, metastatic site, primary tumor sidedness, RAS status, and targeted therapy class. Results: Among 4,445 eligible patients, 263 underwent R0 , 233 underwent non-R0 resection, and 3,949 received no surgical intervention. The R0 resection group comprised patients with liver-only metastases (n = 176), lung-only metastases (n = 23), and combined liver and lung metastases (n = 27). R0 resection was associated with significantly improved OS and PFS compared with both non-R0 resection and no surgery. These survival benefits were consistently observed across key clinical subgroups, including patients achieving complete or partial response to chemotherapy, those with liver or lung metastases, left- and right-sided primary tumors, and both RAS wild-type and mutant disease. Importantly, the benefit of R0 resection was maintained irrespective of anti-EGFR– or anti-VEGF–based first-line therapy. Conclusions: R0 resection following systemic chemotherapy is strongly associated with durable survival benefits in patients with initially unresectable mCRC across diverse clinical settings. These findings support the pivotal role of conversion strategies and highlight the importance of multidisciplinary evaluation to identify appropriate candidates for curative-intent resection.