Although colorectal cancer (CRC) screening guidelines acknowledge the need for earlier screening for high-risk individuals, such as those with family history of CRC, there is limited information on how many years earlier these high-risk individuals should be screened; current practice is based on weak evidence. We aimed to provide risk-adapted recommendations on starting age of CRC screening for individuals with different family histories.
We collected data from nationwide family-cancer datasets in Sweden and calculated risk-adapted starting ages of screening for individuals with different family histories of CRC. Family history was defined as a dynamic (time-dependent) variable allowing for changes during the follow-up period of 1958 through 2015.
During a follow up of 12,829,251 individuals with genealogy information, 173,796 developed CRC. The 10-year cumulative risk for the average-risk population at age 50 y (guideline-recommended age for screening) was 0.44%. Individuals with different family histories of CRC attained this equivalent 0.44% risk 3-32 y earlier than their peers in the general population without such a family history. For example, individuals with 1 affected first-degree relative diagnosed before age 45 reached the corresponding risk level 16 y earlier.
We determined risk-adapted starting ages of CRC screening for close or distant relatives of patients with CRC, using high-quality nationwide datasets. These findings might be used in counselling individuals about the appropriate age to start CRC screening, to optimize screening practice, and to supplement guidelines for CRC screening.

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