The past tense and past participle of broadcast ("to transmit — to cast abroad — programs from a television or radio station") are either broadcast or broadcasted: "The program was broadcast (or broadcasted) at noon." "The senator broadcast (or broadcasted) his appeal yesterday." Because the principal parts of cast are cast, cast, cast, broadcast may sound better to your ear, but widespread usage has made broadcasted an also-acceptable form.
Dictionary of problem words and expressions. Harry Shaw. 1975.