jayhall, I'm out of my league here, but:
when I had cancer ten years ago, I didn't tell anyone. I pretended to go on a month long vacation. I saved everyone the stress, including myself. Once you tell there is a constant. Everyone who knows you has to have the conversation of concern every time you see them, every time you talk to them. Years later you'll still be fielding questions. When you've managed to stop worrying and thinking about cancer there it will be on everyone's lips.
If you decide not to tell her, don't ever tell her. Don't confide in people that might slip her the information, because she will be hurt.
I don't think you get to establish good boundaries with your mom once you tell.