Tomotherapy is a type of radiation therapy in which the radiation is delivered slice-by-slice (hence the use of the Greek prefix tomo-, which means "slice"). This method of delivery differs from other forms of external beam radiation therapy in which the entire tumor volume is irradiated at one time.

The first implementation of tomotherapy was the Corvus system developed by Nomos Corporation. This was the first commercial system for planning and delivering intensity modulated radiation therapy (IMRT). The original system, designed solely for use in the brain, incorporated a rigid skull-based fixation system to prevent patient motion between the delivery of each slice of radiation. But some users^[who?] eschewed the fixation system and applied the technique to tumors in many different parts of the body.

Tomotherapy, or Helical Tomotherapy, is a form of computed tomography (CT) guided IMRT or Intensity Modulated Radiation Therapy, which is a relatively new type of radiation therapy delivery system. The system was developed at the University of Wisconsin–Madison by professor Thomas Rockwell Mackie, Ph.D. and Paul Reckwerdt. A small megavoltage x-ray source was mounted in a similar fashion to a CT x-ray source, and the geometry provided the opportunity to provide CT images of the body in the treatment setup position. Although original plans were to include kilovoltage CT imaging, current models use megavoltage energies. With this combination, the unit was one of the first devices capable of providing modern image-guided radiation therapy (IGRT). The first patients were treated in 2002, at the University of Wisconsin under the guidance of Professor Minesh Mehta, M.D., under the auspices of an NIH-funded Program Project Grant.