A showdown between Tufts Medical Center and Massachusetts' largest healthcare insurer ended when the hospital and Blue Cross reached an agreement that allowed both sides to claim success. Tufts and its affiliated doctors said they will join Blue Cross's "alternative quality contract," which the insurer has been pushing as a way to slow the spiraling cost of healthcare while improving treatment. Under the deal, Tufts and its doctors will get a raise with the potential to earn even more money if they reduce the costs of treatment and reach quality goals.