You shouldn't. Put the shoe on the other foot. Would you think it ethical or right if the hospital or agency just ignored the contract they signed? It is not.
In addition, you will find some pretty stiff penalties in your contract for canceling at any point.
That said, your agency has probably not secured housing this far out, and there are plenty of travelers to take your place. Talk to them. If you have a halfway decent reason (I found a better contract is not one of them) and they are a halfway decent agency, you might be off scott free.