E Entertainment is reporting that she left amicably when the show realized that giving her two months to work on her cancer situation wouldn't be enough. Her part is being recast :(

You can read more here:
http://www.eonline.com/uberblog/index.2.html

I hope Maura will be okay. In addition to being the lovely Lisa Miller (and who among us DOESN'T love Lisa?), she is a very wonderful and talented actress, as well as a beautiful woman with a nice, genuine down-to-earth personality that puts her leagues above the petty "Look-at-me, I'm-a-celebrity!" actresses.

Maura, if you ever read this, please know that we love you very much and wish only the best for your quick recovery. Get well soon! :)

I wish her a speedy and full recovery. She is a talented actress and a good person. I loved her as Lisa, but I didn't care for her ER character at all.