IMRT is intensity modulated radiation therapy. It's designed to limit the radiation delivered to healthy tissue, while giving the tumor the full dose. The radiation source is an x-ray beam that can come at the cancer from several different directions, and can have a different shape from each direction, using something called a collimator that is computer controlled to have the same shape as as the tumor. Think an (American) football-shaped tumor: end-on it looks circular, side-on oval. In IMRT the beam would be circular when coming at the tumor end-on, and oval when the beam direction is side on.