BOSTON — LeBron James scored 21 of his 38 points in the first quarter to help the Cleveland Cavaliers beat Boston 124-95 on Friday night, handing the Celtics their worst home playoff loss ever and taking a 2-1 lead in the best-of-seven Eastern Conference semifinals. Showing no ill-effects of an elbow problem that was the talk of Cleveland during the long layoff, James finished with eight rebounds and seven assists.