Yes, CAST is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, S 1, T 1.

Is it a valid word?

CAST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the actors in a play
container into which liquid is poured to create a given shape when it hardens
the distinctive form in which a thing is made
the visual appearance of something or someone
bandage consisting of a firm covering (often made of plaster of Paris) that immobilizes broken bones while they heal
object formed by a mold
the act of throwing dice
the act of throwing a fishing line out over the water by means of a rod and reel
a violent throw

put or send forth
deposit
select to play,sing, or dance a part in a play, movie, musical, opera, or ballet
throw forcefully
assign the roles of (a movie or a play) to actors
move about aimlessly or without any destination, often in search of food or employment
form by pouring (e.g., wax or hot metal) into a cast or mold
get rid of
choose at random
formulate in a particular style or language
eject the contents of the stomach through the mouth

Also: cast of characters, dramatis personae, plaster cast, plaster bandage, cast off, shake off, throw off, throw away, vomit up, be sick, throw up.