Cancer of the bile duct? If so, its nearly always fatal. By the time the lump is big enough for someone to go to the doctor to get it checked out, its always stage 4, terminal and inoperable. Only way it turns out differently is when a surgeon is poking around in there for some other reason and stumbles across it. Mostly kills people in their 70s and 80s or older so there's not much research done on it.
Same as a friend of mine lasted. Doc gave him 9 months.
EDIT: and I think he was lying a bit about the diagnosis in that letter, or else his docs were lying a bit to him. That may have been what he wanted to believe. If it was discovered "routinely" outside of surgery it was stage 4 to begin with, and there wasn't any hope, and it was probably inoperable (the fact that he got an operation and only lasted 12 months suggests the op didn't do any good).
Not really casting aspersions on him, though, just cancer is really shitty, and this one particularly bad.
The Japanese announcement says 胆管腫瘍.
Just when Nintendo has started having some bright spots in their business.
RIP.