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Tomotherapy Hi Art

Tomotherapy describes 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 radiation 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.[1] This was the first commercial system for planning and delivering intensity modulated radiation therapy (IMRT). The original system was designed solely for use in the brain and incorporated a rigid skull-based fixation system to prevent patient motion between the delivery of each slice of radiation. It was not long before many users 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 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.

General Principles

In general, radiation therapy has developed with a strong reliance on homogeneity of dose throughout the tumor. Tomotherapy embodies the sequential delivery of radiation to different parts of the tumor which raises two important issues. First, this method is known as "field matching" and brings with it the possibility of a less-than-perfect match between two adjacent fields with a resultant hot and/or cold spot within the tumor. The second issue is that if the patient or tumor moves during this sequential delivery, then again, a hot or cold spot will result. The first problem can be overcome, or at least minimized, by careful construction of the beam delivery system. The second requires close attention to the position of the target throughout treatment delivery. The Corvous tomotherapy system achieved great popularity because it provided a mass market solution to IMRT very early compared to other vendors' systems. Generally speaking, dose homogeneity is less in IMRT than in 3D conformal radiation therapy which may account for the relative lack of concern regarding the field matching issue.

Current State

At this time, the HiArt system manufactured by TomoTherapy, Inc. is the primary tomotherapy device in use although there are still a number of Corvus systems being used. Other radiation therapy equipment vendors have recently responded to the challenge of short treatment times coupled with a full 360 degree treatment arc by developing methods of delivering IMRT using arcs. The major difference is that these methods are implemented on standard medical linear accelerators, thereby providing for complete volumetric irradiation.

TomoTherapy "beam on" times are comparable to normal radiation therapy treatment times (about 3–5 minutes beam on time for a common prostate treatment) but do add an additional 2–3 minutes for a daily CT. The daily CT is used to precisely place the radiation beam and allows the operator to modify the treatment should the patients anatomy change due to weight loss or tumor shrinkage (adaptive radiotherapy). Lung cancer, head and neck tumors, breast cancer, prostate cancer, stereotactic radiosurgery and stereotactic body radiotherapy are some examples of treatments commonly performed using TomoTherapy. While the first clinical use of TomoTherapy was in 2002, at the University of Wisconsin, under the leadership of Dr. Minesh Mehta, M.D., there are now more than 200 sites across Canada, the United States, Europe and Asia.

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