GREY'S ANATOMY - "What a Difference a Day Makes" - Denny suddenly reappears to Izzie, telling her that cancer has once again spread to her brain, on "Grey's Anatomy," THURSDAY, MAY 7 (9:00-10:02 p.m., ET) on the ABC Television Network. (ABC/SCOTT GARFIELD) JEFFREY DEAN MORGAN, KATHERINE HEIGL