Over the past 30 years, colon cancer survival rates have steadily improved thanks to advances in prevention, detection, and treatment. Yet recently, researchers have begun to notice an unusual pattern. The tumor’s location — whether it’s on the right or left side of the colon — appears to impact a patient’s survival. A new UC San Francisco study adds data and insight into this matter.

Based on the cases included in their large clinical trial, the investigators say patients with metastatic colon cancer originating on the left side of the colon survive significantly longer than those with cancer originating on the right side. Their research also revealed that a standard colon cancer drug actually provides little benefit to patients with right-sided tumors. Though they do not yet understand why differences exist due to tumor position, they believe colon cancer originating on the right side should be treated differently than colon cancer occurring on the left.

These findings suggest colon cancer is not one disease but a number of different diseases, said Dr. Alan P. Venook, principal investigator of the study. He explained that the right and left sides of the colon arise from different tissue in the embryo, so each has a separate biology.

Previous research suggested that tumor location could affect clinical outcomes, but the effects we observed in this trial appeared to be far greater than we expected,” Venook said in a statement.

Researchers in Switzerland, Germany, Taiwan, and Canada had all previously found lopsided survival rates — colon cancer patients with left-sided tumors generally had a better prognosis than those with right-sided tumors.

Colon Basics

The colon or large intestine is the final part of the digestive system. Its job is to remove water, salt, and nutrients from undigested food, which it then transforms into stool. The right — or ascending — colon travels up the right side of the abdomen, connecting to the transverse colon, which runs across the abdomen. Next, the left or descending colon travels down the left abdomen where it meets the sigmoid colon, a short curve just before the rectum. The colon's walls are lined with muscles that squeeze the contents along.

Generally, polyps and cancers of the colon appear on the left side. The cancer often grows around the colon wall and encircles it, constricting the channel and causing a partial blockage. Symptoms typically include constipation, changes in bowel habits, and, when a cancer is low in the rectum, narrow, ribbon-shaped stool. By contrast, right-side tumors usually grow into the space within the colon. If they become large enough to be painful, they also are likely to cause bleeding.

Launched in 2004, the current study involved 293 patients with right-sided primary tumors and 732 patients with left-sided primary tumors. Men comprised more than half of the patients. The median age of patients with right-sided tumors was 61, while the median age of patients with left-sided tumors was 57.

Overall, those with cancer on the left side survived for an average 33.3 months. By contrast, patients with right-sided tumors averaged 19.4 months survival. Additionally, left-sided patients treated with cetuximab added to their first-line chemotherapy averaged 36 months survival, while right-sided cancer patients on the same drug lived for about 16.7 months.

Venook says this difference is significant. Though more research is needed to support these results, he says the side of the primary tumor undoubtedly indicates biological characteristics, even if these traits are not yet understood. Until scientists comprehend the underlying biology, right-sided colon cancer should be considered different than left-sided colon cancer.

Source: Venook AP, Niedzwiecki D, Innocenti F, et al. Impact of primary (1º) tumor location on overall survival and progression-free survival in patients with metastatic colorectal cancer: Analysis of CALGB/SWOG 80405. American Society of Clinical Oncology. 2016.