Btw, "Close" and "Back" buttons should have the order reversed, so its "Back" and then "Close". It's just the logic of operation. I am always confused which one I should press.
As for implementing the OP request, maybe 3 radio buttons:
Code:
After render do: o Nothing o Go back O Close
Notice the total string length is not too much longer than the original checkbox and is clear enough IMO.