Early on in his treatment he mentioned going with some kind of experimental approach at treating it.
He’s only shared what he’s chosen to share and doesn’t want to put too much out there (even the particulars of the type of cancer that he has).
Because he’s a cigar smoker, and this sounds like an aggressive cancer, I’m going to guess it’s esophageal.
I believe that earlier treatment made his legs swell up, and the pain was unbearable, so it was stopped. I don’t get to listen to Rush much, but follow the threads about him here.