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Family history of colorectal cancer (CRC) in a first-degree relative is associated with substantial excess risk for CRC. However, risk associated with having family history of CRC in a second-degree relative, a designation that includes half-siblings, is unclear. In this Swedish study of 13 million people with mean follow-up of 34 years, CRC risks conferred by family history were as follows:
Similar excess risk for CRC was found for people who had one CRC-affected first-degree relative or a CRC-affected half-sibling (standardized incidence ratio [SIR], 1.5).
Having two CRC-affected first-degree relatives or one affected first-degree relative plus one affected half-sibling also led to similar excess risk (SIRs, 2.7–3.6).
Having one affected sec…