If it were a case where you were having symptoms but not already getting medical care, I could see him saying something like, "There's a chance it could be leukemia, so you should be checked out." (or at least that it could be something serious). But because you're already getting treatment for it, he really shouldn't mention random things. Yes, he does have a medical degree, but I doubt he has training in oncology. So I agree it's irresponsible--plus, maybe it could scare you needlessly.