Metastatic colorectal cancer with the primary tumor located on the left side of the colon is associated with longer survival compared with cancer originating on the right side, a retrospective analysis that will be presented at the 2016 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago, IL, has shown.1

The phase 3 CALGB/SWOG 80405 study demonstrated no significant difference in overall or progression-free survival when bevacizumab or cetuximab was added to frontline FOLFOX (fluorouracil, leucovorin, oxaliplatin) or FOLFIRI (fluorouracil, leucovorin, irinotecan) in patients with metastatic colorectal cancer. Researchers sought to evaluate whether the location of the primary tumor affected clinical outcomes.

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For this analysis, researchers assess data from 293 patients with right-sided primary tumors and 732 patients with left-sided primary tumors. Only patients without a KRAS-mutant gene were included, as it is a known biomarker of response to cetuximab and other colorectal cancer therapies.

Results showed that median overall survival was 33.3 months for patients with left-sided tumors compared with 19.4 months for patients with right-sided tumors.

Researchers found that among patients who received cetuximab, median overall survival was 36 months and 16.7 months for those with left-sided tumors and right-sided tumors, respectively. A similar trend was observed among patients who received bevacizumab (31.4 vs 24.2 months, respectively).

“While previous studies had suggested that tumor location may impact clinical colorectal cancer outcomes, the effect we observed in this analysis appears to be far greater than we expected,” said lead investigator Alan P. Venook, MD, professor of medicine at the University of California, San Francisco. “These findings will likely change the way we approach colorectal cancer treatment and research, even as we seek to more deeply understand the biology driving the difference in outcomes between right- and left-sided cancers.”

The study further demonstrated that among patients with right-sided tumors, bevacizumab treatment was associated with a median survival of 24.2 months vs 16.7 months with cetuximab. In contrast, cetuximab treatment conferred a median survival of 36 months vs 31.4 months with bevacizumab among patients with left-sided tumors.

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“This is the largest study to date of tumor location in colorectal cancer, and it strongly suggests that this unexpected factor could answer some long-standing questions about why certain patients do better than others,” said ASCO President Julie M. Vose, MD, MBA, FASCO. “It is also an important reminder, in this exciting era of precision medicine, that genomics is not the only source of insight into how cancers should be studied and treated.”                                      


  1. Left vs. right: Primary tumor location predicts survival in metastatic colorectal cancer [news release]. Alexandria, VA: American Society of Clinical Oncology; May 18, 2016. Accessed May 18, 2016.