CHICAGO (Reuters) – Colon cancer patients with a family history of the disease may live longer once treated than those without the family link, researchers said this week.

The reason is not clear, said Dr. Jennifer Chan and colleagues at Dana-Farber Cancer Institute in Boston, but it may be that tumors in family clusters are of a type that behave differently compared to those that are random.

Having a parent or sibling with colon cancer results in twice the risk of the disease, compared to those without a family history.

But less is known about survival rates in such families, the authors said, and previous studies have yielded conflicting results.

The findings published in this week's Journal of the American Medical Association came from a look at 1,087 US colon cancer patients who had undergone surgery and chemotherapy. Nearly 18 percent of the group said they had one or more parents or siblings with a history of the disease.

Those with a family history had a 28 percent lower risk of the cancer returning, being diagnosed with a new tumour, or dying from any cause during the five-and-a-half years they were followed, compared to others in the group.

"This news may be reassuring to people with a family history, but our hope is that we can discover what underlies this effect ... in biological terms," Chan said.

Colon cancer kills 50,000 people each year in the United States alone.

While people with a family history are told to undergo exams more often, the closer monitoring did not appear to cause the better survival rate – nor did other factors such as diet – exercise and smoking, the research team said.