By Lisa Rapaport

(Reuters Health) – – Siblings of people with colorectal cancer are themselves at higher risk for the disease – but a new study suggests half-siblings face nearly as a high a risk.

Colorectal cancer in first-degree relatives – parents, children and siblings – is strongly associated with an increased risk of these tumors. Some studies have also found a smaller risk increase in second-degree relatives, such as grandparents, grandchildren, aunts, uncles, nephews, nieces and half-siblings, but others have not found this link in these more distant relatives.

For the current study, researchers analyzed data on more than 16 million people living in Sweden from 1958 to 2015. Among people with clear genealogy records, 173,796 developed colorectal cancer.

Siblings of colorectal cancer patients had a 7 percent cumulative lifetime risk of developing these tumors, researchers report in The BMJ. That is 1.7 times the risk of colorectal cancer for people without any family history of these tumors.

Half-siblings of colorectal cancer patients had a 6 percent cumulative lifetime risk, which is 1.5 times the risk without a family history.

“We showed that family history of colorectal cancer in a half sibling (with no other affected first/second-degree relative) has a much stronger association with increased risk of colorectal cancer than such a family history in other second-degree relatives,” said senior study author Dr. Mahdi Fallah of the German Cancer Research Center and the National Center for Tumor Diseases in Heidelberg.

“While from genetic point of view, half siblings are second-degree relatives, we found that from familial risk of colorectal cancer perspective, they are closer to first-degree relatives than to other second-degree relatives,” Fallah said by email.

Colorectal cancer is the third most common cancer and the second leading cause of death from cancer worldwide, the study authors write.

An estimated 70 to 80 percent of the risk of these tumors depends on the level of poverty or affluence where people live, as well as other environmental factors like social and cultural traditions and lifestyle, researchers note. About 2 to 5 percent of risk depends on genetics, but the remaining causes of inherited colorectal cancers are unknown.

People with both a parent and a half-sibling with colorectal cancer in the study had a 3.6-fold higher risk of developing these tumors than individuals without any family history of the disease.

When both a parent and sibling had colorectal cancer, people had a 2.7-fold greater risk of developing these tumors, too.

There wasn’t a meaningful increase in the risk of colorectal cancer when people had only one second-degree relative other than a half sibling develop these tumors, however.

“While it is likely that siblings and half-siblings have an increased colorectal cancer risk due to genetics, the extent to which environmental factors, such as shared life experiences and lifestyles, play a role is unclear,” Joshua Demb, a researcher with the Moores Cancer Center at the University of California, San Diego and the San Diego Veterans Affairs Healthcare System.

“This interplay of risk factors makes up a large portion of current research,” Demb, who wasn’t involved in the study, said by email.

Still, the results suggest that guidelines recommending earlier colorectal screening only when first-degree relatives get this type of cancer might be overlooking the risk associated with half siblings, Demb added.

“This study provides strong evidence that individuals with a half sibling previously diagnosed with colorectal cancer have a higher risk of colorectal cancer than individuals without any family history,” Demb said.

SOURCE: https://bit.ly/2HVoxVj The BMJ, online March 14, 2019.