To make any kind of joke about cancer victims is both thoughtless and insensitive. Everyone surely knows someone who has cancer or has died from cancer and it only harbours a particular kind of sadness for all concerned. Granted that many jokes have been made about disasters... which I think is insensitive anyway.. to all concerned.... but some are taken as at least acceptable and actually help people to come to terms by adding humour.
Sorry but cancer is not one of those disasters or occurrences which merits any kind of humour..... only sadness, pity, fear, hurt etc etc.
Best wishes, Mike.