Background: There is currently no consensus about whether bevacizumab effectiveness is associated with the primary tumor location of metastatic colorectal cancer (mCRC). The aim of this study was to assess whether the primary tumor location was a predictor for bevacizumab treatment. Methods: From 2004 to 2013, 740 patients with mCRC treated with oxaliplatin / 5-FU / leucovorin (mFOLFOX6) or irinotecan / 5-FU / leucovorin (FOLFIRI) (CT group) and 244 patients treated with bevacizumab plus mFOLFOX6 or FOLFIRI (CT + B group) as first-line setting were included from Sun yat-sen university cancer center. Right-side colon cancers included those occurring in the cecum, ascending colon or transverse colon. Left-side colon cancers included those from descending or sigmoid colon. The primary outcome was overall survival (OS). Kaplan-Meier curves with log-rank tests were used to detect difference. All statistical tests were two sided. Results: 222 right-side colon, 259 left-side colon and 259 rectal cancer patients were included in CT group while 78 right-side colon, 86 left-side colon and 80 rectal cancer patients were included in CT + B group. Patients in CT + B group had similar OS compare with CT group only when the primary tumor located at right-side colon (median OS was 19.6 months for CT + B group versus 19.5 months for CT group, P = 0.269). For left-side colon cancer, significantly longer OS were observed in CT + B than CT group (22.3 months versus 21.9 months, P = 0.014). For rectal cancer patients, those in CT + B group also had longer OS than CT group (25.9 months versus 21.1 months, P = 0.005). Conclusions: Our data suggested that patients with right-side colon cancer could not get survival benefit from the addition of bevacizumab to first-line chemotherapy. Further data from randomized trials are needed to test our hypothesis.

HR= hazard ratio; CI= confidence interval.