1. A method for reducing variation in radiation therapy treatment planning among a plurality of users, comprising the steps of: providing a system which contains a knowledge information; providing a workstation which is configured to provide the users with the knowledge information received from the system; providing a processor which is configured to use the knowledge information to contour target volumes for a radiation treatment planning; whereby the target volumes will be contoured by the users according to the knowledge information as references such that two or more of the users provide consistent treatment planning.