With many of my interviews I was able to call them and cancel, but a few of the schools wanted an e-mail from me confirming my decision to be withdrawn from the application cycle. But you should cancel as soon as you know that you are going somewhere else. You never know when you might want to rotate through one of their hospitals and they see that you applied but only gave them a week to fill your interview spot. Just do it as soon as possible. See you next year at KCOM