| Summary: | Background: The decision to resect or not the primary tumor in asymptomatic patients with synchronous metastatic colorectal cancer (mCRC) is a complex and challenging issue for oncologists, especially when an antiangiogenic-based therapy is planned. Methods: Patients enrolled in the phase III TRIBE and TRIBE2 studies that compared upfront FOLFOXIRI + bevacizumab to FOLFIRI or FOLFOX + bevacizumab, respectively, were included. We assessed the association of primary tumor resection (PTR) with progression-free survival (PFS), overall survival (OS), response rate (ORR), rate of grade > 2 adverse events (AEs), and serious gastrointestinal and surgical AEs in the overall population and according to the treatment arm. Results: Of the 999 patients included, 513 (51%) underwent PTR at baseline. Longer PFS and OS were observed in resected patients compared to those with unresected primary tumors: 11.2 vs. 10.0 months (<i>p</i> < 0.001) and 26.6 vs. 22.5 (<i>p</i> < 0.001), respectively. In multivariate models, PTR was confirmed as an independent prognostic factor for better PFS (<i>p</i> = 0.032) and OS (<i>p</i> = 0.018). Patients with PTR experienced a higher incidence of grade 3 or 4 diarrhea (<i>p</i> = 0.055) and lower incidence of anemia (<i>p</i> = 0.053), perforation (<i>p</i> = 0.015), and serious gastrointestinal and surgical AEs (<i>p</i> < 0.001). No statistically significant differences were noted in incidence of bleeding (<i>p</i> = 0.39). The benefit of FOLFOXIRI + bevacizumab in terms of PFS (<i>p</i> for interaction: 0.46), OS (<i>p</i> for interaction: 0.80), ORR (<i>p</i> for interaction: 0.36), and incidence of grade 3 or 4 AEs was independent of PTR. Conclusions: PTR at baseline was independently associated with good prognosis in synchronous mCRC patients and with lower incidence of serious gastrointestinal and surgical AEs during upfront chemotherapy plus bevacizumab. The benefit and toxicity profile of FOLFOXIRI plus bevacizumab was independent of PTR.
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