1. A method for facilitating target volume contouring in conformal radiation therapy planning using interactive expert guidance, comprising providing a first patient's volumetric imaging data; providing expert's volumetric imaging data derived from a group of second patients with expert's contours of the group of second patients having similar disease nature or clinical manifestation; and contouring target volumes on first patient's imaging set in a slice-by-slice step using expert's guidance with the guidance derived from the second group of patients.