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rate (ORR), TTP, and OS: RAS (KRAS/NRAS) mutational status, primary tumour location and metastases site, efficacy of first-line treatment, sex, age, body mass index, lactate dehydrogenase (LDH), alkaline phosphatase, CEA, lymphocytes and haemoglobin level at the time of beginning second line treatment. Median follow-up was 35 months. Results: In whole group median TTP was 5.1 months and OS -12.9. Response rates: CR -1%, PR -18%, NC -40%, PD -40%. For patients who achieved ORR (CR or PR) median TTP and OS was 7.8 and 19.3 months while for patients with PD -2.8 and 9.9. Hepatic location of metastases was associated with the highest rate of ORR, while peritoneal with the lowest: 46% and 0% (p ¼ 0.041) respectively. Median TTP for patients who gained CR, PR or NC (DCR -disease control) while first-line treatment was 6.7 compared with 3.3 months for patients with PD (p < 0,0001). TTP under/equal 6 months and PD during first-line treatment were associated with shorter OS: 12.9 vs 15.3 months (p ¼ 0.049) and 9.9 vs 14.2 months (p ¼ 0.007). Age over 70 and BMI !25 were good prognostic factors with OS 19.6 months vs 10.4 (p ¼ 0.01) and 18.4 months vs 9.1 (p ¼ 0.039) respectively. There was no difference between patients in terms of KRAS and NRAS mutational status. For wild type and mutant tumours it was 6.6 months vs 6.1 (p ¼ 0.098) and 17.9 vs 12.9 (p ¼ 0.183) respectively in TTP and OS. Right-side primary tumours were associated with worse DCR, TTP and OS compared to left-side primary; 45%, 2.3 months and 11.2 months vs 61%, 4.9 months and 12.8 months, but the differences were not statistically significant. Other factors did not occur relevant. Sixteen patients (20%) discontinued treatment due to adverse events. There were no toxic deaths. Conclusion: Positive prognostic impact on second-line treatment with bevacizumab and FOLFOX4 in mCRC have: first-line treatment response, hepatic site of metastases, age over 70 and BMI !25. Worse prognosis was observed in group of patients with peritoneal metastases and poor response for first-line chemotherapy. RAS mutational status seemed to not influence prognosis in the analysed cohort. Location of the primary tumour in the right side of the colon was not proven to be a negative prognostic factor. P À 231 Clinical significance of microsatellite instability in Introduction: Colorectal cancer (CRC) with microsatellite instability (MSI) are known to have better prognosis compared to those with microsatellite stable (MSS). Recent studies reported that there are biological differences according to tumor location in CRC. In this study, we aimed to identify the clinical significance of MSI in patients with right-sided CRC. Methods: Between October 2004 and December 2016, medical records from a total of 1,009 patients with CRC were retrospectively reviewed. Patients with MSI testing were included in the analysis. We assessed the long-term outcomes of MSI according to the tumor location using the Kaplan-Meier curves and Cox regression models. Results: The median follow-up duration was 25 ...

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842 Background: The standard adjuvant treatment for patients with stage III colon cancer is 6 months with fluoropyrimidines and oxaliplatin. The Intergroup Trial INT-0035 was the first large-scale study to demonstrate a significant reduction in the risk of death with adjuvant FU plus levamisole in patients with stage III colon cancer. In the MOSAIC study, the addition of oxaliplatin to fluoropyrimidines in patients with resected stage II to III colon cancer showed OS and DFS benefit of oxaliplatin. However, no significant benefit was observed in either DFS or OS in patients with stage II disease, therefore the benefit of adjuvant chemotherapy is still controversial in those patients. Methods: We retrospectively included patients with stages II and III colon cancer that were operated between 2009 and 2014 in the University Ramón y Cajal Hospital from Madrid. We calculate DFS and OS at 48 months and we perform a multivariable Cox model analysis to estimate the benefit of the chemotherapy in each stage. The model was further adjusted by including the following confounders: ECOG-PS, number or removed nodes ( < 12 or ≥ 12), grade and age. A covariate was considered a confounder factor if the difference between the adjusted and unadjusted coefficient of chemotherapy varied > 10%. Stata 13.1 was used to analyze the data. Results: 564 patients were identified (281 stage II and 283 stage III). 259 received chemotherapy and 305 did not. The median follow-up in the entire cohort was 49 months. The median DFS and OS were not reached at the moment of the analysis. DFS and OS at 48 months were both 78.5%. Globally, chemotherapy did not improve DFS (HR 1.05, p: 0.83) but OS was significantly better (HR 0.47, p: 0.001). By stage, chemotherapy did not improve DFS in stage II (HR: 1.6, p: 0.2) nor OS (HR 0.76, p: 0.43). In stage III, chemotherapy showed a trend to improve DSF (HR: 0.61, p: 0.075) and did improve OS (HR: 0.31, p < 0.0001). Conclusions: The multivariable analysis showed a chemotherapy benefit in patients with stage III colon cancer, with a 39% reduction in the risk of recurrence and a 69% in the risk of death; however, in stage II patients these benefits were not found either in DFS or OS.