Blue Cross Blue Shield of Massachusetts has ratcheted up the fight with the Tufts Medical Center by ending negotiations with the medical center and 1,500 physicians ? potentially disrupting care for more than 200,000 patients across Eastern Massachusetts. Tufts Medical officials said Blue Cross walked away from the negotiating table last night after refusing to review a proposal from the medical center. A Blue Cross spokesman denied the healthcare insurer walked away from the negotiations. "Tufts is the one that has made a decision to terminate their contract with us and no longer care for our members effective January 17, 2012," said Blue Cross spokesman Jay McQuaide. But Tufts Medical officials blamed Blue Cross.