A retrospective analysis of data from the CALGB/SWOG 80405 clinical trial revealed that in patients with metastatic colorectal cancer, the location of the primary tumor predicts the patient’s response to various treatment strategies for the disease.

Patients with left-sided primary tumors responded significantly better to cetuximab, which had an average survival of 39 months as opposed to 32 months with bevacizumab. Conversely, patients with right-sided primary tumors responded more favorably to bevacizumab.

Heinz-Josef Lenz, MD, J. Terrence Lanni Chair in Gastrointestinal Cancer Research and professor of medicine at the Keck School of Medicine of USC, chair of the Gastrointestinal Oncology Program at USC Norris Comprehensive Cancer Center and co-chair of the SWOG 80405 clinical trial, presented the data at the European Society for Medical Oncology on Oct. 10. He also was a co-author on a paper published in JAMA Oncology online that illustrated similar findings through the CRYSTAL and FIRE-3 clinical trials.

“The molecular differences between left and right-sided tumors in the colon are such that they are essentially two different diseases,” Lenz said. “More research is needed to determine why there is such a vast prognostic difference; but for the time being, tumor sidedness should be heavily considered when developing a treatment plan for metastatic colorectal cancer patients, and future clinical trials should first stratify participants by primary tumor location.”

By Mary Dacuma