I just read your Cedric Benson story on the KC Star website, which concludes with this:
While Benson suffered from a variety of lower-body injuries in the NFL, there are no records of him ever suffering a concussion.
We believe CTE is caused by repetitive brain trauma. This trauma includes both concussions that cause symptoms and subconcussive hits to the head that cause no symptoms. At this time the number or type of hits to the head needed to trigger degenerative changes of the brain is unknown.