GREY'S ANATOMY - "Scars and Souvenirs" - The race for chief heats up after a new competitor enters the fray, tensions escalate between Izzie and George, and Callie must reveal a big secret. Meanwhile, Derek treats a patient near and dear to him, while Alex continues his work with Jane Doe, on "Grey's Anatomy," THURSDAY, MARCH 15 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/RON TOM) PATRICK DEMPSEY, T.R. KNIGHT, SHOHREH AGHDASHLOO