When I started medical school 50 years ago, rectal cancer was common, so we did sigmoidoscopies to screen, but now the cancer has changed and is more common upstream, which needs a colonoscopy to diagnose.
the theory is that the transit of feces is faster in a high fiber diet, so the toxins don't infect the tissue as much. And with the increase in fiber in American diets, this might explain the evolution of the cancer.
However, I suspect some rectal cancers might be from the same pappilloma virus that causes cervical (but not uterine) cancer.
Thanks for the good info!