A type of therapy in which radiation is aimed at a tumor from many different directions. The patient lays on a table and is moved through a donut-shaped machine. The radiation source in the machine rotates around the patient in a spiral pattern. Before radiation, a 3-dimensional (3-D) image of the tumor is taken. This helps doctors find the highest dose of radiation that can be used to kill tumor cells while causing less damage to nearby tissue. Tomotherapy is a type of intensity-modulated radiation therapy (IMRT). Also called helical tomotherapy.