Complex sentence: On November 13th, 2008, over 6 years after his exit from the show, Anthony Edwards returned as Dr. Mark Greene to treat Catherine Banfield's son Darryl who had a debilitating disease which turned out to be leukemia.
Split into simpler sentences: On November 13th, 2008, over 6 years after his exit from the show, Anthony Edwards returned as Dr. Mark Greene. He was treating Banfield's son Darryl who had a debilitating disease which turned out to be leukemia.