"Hi, How are you, " you say cheerfully, "I am really fine at the moment". Then there is that awful feeling as you say to yourself "That was a bit tactless - OMG what do I say to him/her?" You make your embarrassed apologies and run for the hills.
Or if you have thought a bit, you can say something equally earth open up and swallow me like "I've heard you are undergoing chemo - I'm sure that will turn out fine" or "I hope you'll be well again soon"
I expect this is what happens to a lot of us when faced with this, because we have never thought about what should we say. Perhaps we get too sensitive, when actually the sufferer would only be too pleased that somebody is concerned and talk to them, not running away because they don't know what to say.
I try and think of questions to ask like "I've heard your news - is it OK to talk about it?" or "How is your treatment going?" or "Is there any support that we can give to help you?" but I still admit, I find it difficult as some people do not want to talk about it or even, sometimes, let other people know.
Just a few thoughts but I expect a lot of us would like some thoughts from those trying to deal with this for friends, family or themselves as to what would make it easier for them to hear, so we don't make such a mess of it as we can sometimes do!