GREY'S ANATOMY - "Freedom" - On the two-hour season finale of "Grey's Anatomy," Meredith and Derek have one last shot at a successful outcome in their clinical trial, as the other surgeons work together to free a boy from a hardening block of cement. Meanwhile, Izzie helps Alex care for an ailing Rebecca, and Lexie discovers critical information about George's intern status, on "Grey's Anatomy," THURSDAY, MAY 22 (9:00-11:00 p.m., ET) on the ABC Television Network. (ABC/MICHAEL DESMOND) ELLEN POMPEO, MARSHALL ALLMAN