Hi Lynn,
I'm open with people I regularly see or talk to, because I don't feel comfortable trying to pretend nothing is wrong when there is. That has backfired on occasion because some people then fixate on my cancer, and I have to continually remind them that I can talk about something else, and, in fact, most times would prefer to. I don't mind the occasional request for an update, or show of concern. But I have coworkers who bring it up everytime they see me. I've had to say something like "Let it go, already. I wanted you to know that I'm dealing with this, but I'd like us to talk about other things, too." So do what makes you most comfortable, but be prepared that some people have a morbid curiosity that you may then have to deal with.
If I prefer not to share my health situation with someone, I give a vague reply like "Oh, things could be better but things could be worse." At least I'm not saying that big phony "FINE! Everything's going great." I have a face you can read like a book and can't pull that off anyway.
When I had my 2nd recurrence last year, I at first didn't tell any acquaintances or casual friends, because I didn't want to be treated differently. But it became difficult because it was affecting me too much. Medical care took me away from work, medical bills limited my social life, and people were wondering where I was and what was wrong. I felt better once I told them.
I wouldn't say it gets much easier with time (I've been dealing with bladder cancer for over 9 years), but I've gotten used to it.
Best of luck to you.
Connie