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What surprising to me is that sigmoidoscopy shows a benefit but colonoscopy doesn’t.

In colonoscopy, the entire colon is examined by using a long flexible tube. In sigmoidoscopy, only the lower portion of the colon is checked, again by using a flexible tube.

The things that come to mind is that sigmoidoscopy may be better tolerated by patients or have fewer complications.

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Likely that the cancers found by the sigmoid are the ones that reduce life years. Perhaps the cancers that are found and treated higher up the colon would have been found out in other ways or not serious enough? I'm not a doctor, so not sure.

Indeed there are dramatic differences between ascending and descending colon cancers.

They are also dissimilar in their chances of presenting early with symptoms making screening less useful vs those that don't present early.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6089587/

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