Helical tomotherapy is a volumetric image-guided, fully dynamic, intensity-modulated radiation therapy (IMRT) delivery system. The daily use of its pretreatment megavoltage (MV) CT imaging for patient setup verification allows one to correct for interfraction setup error. This is a primary requirement for the accurate delivery of complex IMRT treatment plans, which give differential radiation doses to various target volumes while conformally avoiding normal critical structures. In particular, image guidance using MV CT allows for direct target position verification with the patient in the actual treatment position just prior to therapy delivery. Moreover, since helical MV CT imaging is a slow CT imaging technique, it allows for the encoding of target motion in the resulting MV CT data set, and therefore the pretreatment verification of a motion envelope defined from four-dimensional CT.