I worked in a lab that was studying prostate cancer for a summer internship, and most of our samples came from young men who died in motorcycle accidents. They found that a majority of the samples had some presence of benign tumors, but it only really becomes noticeable once malignant tumors develop.
I don't know how representative the samples were of the entire population - one theory was that our young motorcycle samples had significantly higher testosterone on average and were more likely to develop those tumors.
