Killing someone with a poisoned umbrella (quick, effective and hopefully discreet although not as it turned out, and a murder known to have been committed) is one thing. Giving them cancer, quite apart from anything else, seems a bizarre method of killing someone. Then there's the question: "why would they do that? Hugo Chavez might have irritated the US, but well, need I go on? It's just another idle but predictable conspiracy theory with not a shred of evidence to back it up.
The Castro cigar thing: would Castro have seemed ridiculous if his cigar had exploded? I don't think so: we're not talking Red Nose day pranks here, we talking about trying to eliminate someone effectively and discreetly (see above)
It would be hard to think of a more high-profile and obviously incriminating way to do it. I can imagine it being suggested, and I have no doubt the CIA could have done it, but .. it never happened apart from anything else...