Patients with advanced colon cancer have a worse prognosis if their tumor began on the right side of the colon rather than the left, according to a study being presented next month at the annual meeting of the American Society of Clinical Oncology. Right-side colon cancers — meaning the right side from the patient’s perspective — are also less likely to be helped by precision-medicine drugs.

“We have to take that into account when we treat patients,” said Dr. Alan Venook of the University of California, San Francisco, who led the study. “Our results are very compelling that left-side colon cancers are different from right-side ones.”

The study is based on data from a federally-funded clinical trial that was launched in 2004 to compare how patients with colorectal cancer that had spread (usually to the liver, lungs, or bones) responded to the addition of molecular-targeted treatments to standard chemotherapy. As Venook and his colleagues reported in 2014, there was no difference between adding bevacizumab (sold by Genentech as Avastin) or cetuximab (sold by Eli Lilly as Erbitux) to standard chemotherapy in terms of patients’ survival or how long it took before their cancer worsened.

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But because a couple of small studies had suggested left- and right-side colon cancers might be different, Venook decided to pull the medical charts of all 1,137 patients from the completed trial and see if side mattered.

Patients whose tumor began on the right side (293 people) had a median survival of 19.4 months (16.4 with Erbitux and 24.5 with Avastin). Median survival means half the patients lived longer, half less than that. Those whose tumors began on the left (732 patients) had a median survival of 33.3 months (37.5 months on Erbitux, 32.1 months on Avastin).

Presentations at ASCO are not vetted as thoroughly as papers published in a peer-reviewed journal, and typically much less data are available for experts to assess. Nevertheless, physicians not associated with the study said its findings seemed sound and made sense biologically.

“We can’t think of all colon cancers as the same,” said Dr. Nancy Baxter, a surgeon at St. Michael’s Hospital in Toronto who led a 2009 study finding that colonoscopy was less effective at finding cancers on the right side and therefore reduced mortality only from left-side tumors. Polyps on the right side tend to be flatter — they’re stuck to the colon like a barnacle rather than swaying from a stalk like a mushroom — and therefore harder to detect.

But that would not explain why metastatic colon cancer responds to drugs differently depending on which side of the colon it came from. More likely that’s because the left and right sides of the colon develop from different parts of the embryo. “They’re different organs when they start out and then merge to form one,” Baxter said. “So it makes sense that they, and the cancers that develop in them, are biologically different, too.”

The molecular profiles of left- and right-side colon cancers are probably different enough to make them “different cancers,” Venook said. “Colon cancer that originates on the right side should be treated differently from colon cancers that originate on the left.”

For instance, if other studies confirm these results, it might not make sense to treat right-side cancers with Erbitux, especially given the drug’s serious side effects. But those cancers may benefit more from Avastin.

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