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Tomotherapy or helical tomotherapy (HT) 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"). HT is a form of computed tomography (CT) guided intensity modulated radiation therapy (IMRT). HT machines are purpose built for IMRT and differ from IMRT delivered by conventional medical linear accelerators (LINACs) in a number of ways. The main difference is that in HT a narrow intensity modulated pencil beam is delivered from a rotating gantry while the patient is simultaneously moved through the bore, compared to the much wider intensity modulated beam and static patient in conventional IMRT. HT units are therefore better able to target treatment sites throughout the body without a pause for the patient to be moved and set-up differently.
The tomotherapy technique was developed in the early 1990s at the University of Wisconsin–Madison by Professor Thomas Rockwell Mackie 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 implementation of tomotherapy was the Corvus system developed by Nomos Corporation, with the first patient treated in April, 1994. 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.
In 1988 two wisconsin scientists spent night after night performing stereotactic radiosurgery for cancer patients, using technology which although state-of-the-art and cutting-edge then, would be regarded as crude by today’s standards. One was a young physicist from Canada, T Rockwell Mackie, who grew up in the town where Cobalt therapy, a form of radiation treatment, was developed; Minesh Mehta was the other.
Eating pizza between seeing patients, the duo brainstormed about a new approach to radiation therapy, one that would target tumors more precisely and not damage so much healthy tissue at the same time. Initially, in collaboration with a small number of brilliant innovators, this led to the creation of software for 3D radiotherapy and stereotactic radiosurgery, resulting in the development of the early versions (initially referred to as “UW Stereo”) of what is possibly the most widely used software for 3-D radiotherapy and IMRT, PinnacleTM, currently marketed by Philips. But on a grander scale, they dreamt of a CT scan-like device that would also let doctors pinpoint radiation precisely to the tumor.
Rock Mackie established a brilliant team that conceptualized what was then thought of as a “crazy idea”, now known as TomoTherapy—the first-of-its-kind technology, a fully integrated and dedicated intensity-modulated image-guided radiotherapy device which has grandly contributed to a major transformation in radiation oncology.
Standard radiation therapy begins with a CT scan to delineate the tumor and normal tissues and to provide anatomic information for treatment planning and dose calculation. A small number of uniform beams of radiation (typically 2 to 6), target the tumor, often resulting in irradiation of large volumes of normal tissue to high doses. Further, the imprecision in daily reproducibility necessitates relatively large margins to avoid a “geographic miss” and these large margins further increase the risk of damage to normal tissues.
TomoTherapy, solves most of these limitations in an integrated and dedicated device, purpose-built to radically alter conventional radiotherapy. The ability to rotate the machine gantry completely around the patient while indexing the couch through the bore of the Computed Tomography (CT) allows the creation of a spiral pattern of dose-distribution from hundreds of beam directions, and the ability to modulate each of these tiny beamlets “on the fly” with a specially designed and fabricated device allows for the creation of dose-distributions that mimick the three-dimensional structure of the tumor, while elegantly sparing normal tissues.
The need for large margins is obviated because each treatment begins with a megavoltage CT scan, fused to the original treatment-planning CT scan, to verify accuracy on a daily basis. The ability to collect “exit dose” on CT detectors further permits daily verification of radiation dose-delivery, and the serial availability of both changes in tumor anatomy and variations in dose-deposition permits “adaption” of radiotherapy treatment plans. TomoTherapy therefore makes it possible to deposit a dose of radiation that reproduces the exact shape of the tumor, so the surrounding normal tissue gets far less high dose exposure to radiation.
Accurate targeting also means that doctors can safely deliver more radiation dose per fraction and potentially shorten treatment duration. This has made it possible to reduce the standard eight- to nine-week treatment for prostate cancer to four to five weeks (and even as short as 2 weeks) for some patients and lung cancer from eight to 10 weeks to five weeks or even shorter; the biologic advantages of such schedule shortening have been prospectively tested in a series of clinical trials developed and led by Drs. Mehta, Mackie, and colleagues.
Over 125 radiation centers around the world already use TomoTherapy. In the words of Christopher J. Schultz, Radiation Oncology Professor at Medical College of Wisconsin, "It's the next generation of radiation therapy treatment machines".
In general, radiation therapy (or radiotherapy) 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 is reduced by use of a helical motion, as in spiral computed tomography. The second requires close attention to the position of the target throughout treatment delivery.
At this time, the Hi-Art system manufactured by TomoTherapy Inc. is the primary tomotherapy device in use although there are still a number of Corvus systems being used. TomoTherapy TomoHD systems are also in use.
TomoTherapy "beam on" times vary compared to normal radiation therapy treatment times (HT irradiation time can be as low as 6.5 minutes for 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 patient's anatomy change due to weight loss or tumor shrinkage (Image-guided radiation therapy). Lung cancer, head and neck tumors, breast cancer, prostate cancer, stereotactic radiosurgery(SRS) and stereotactic body radiotherapy (SBRT) are some examples of treatments commonly performed using tomotherapy. Some research has suggested HT provides more conformal treatment plans and decreased acute toxicity.
There are few head to head comparisons of HT and other IMRT techniques, however there is some evidence that VMAT can provide faster treatment while HT is better at sparing surrounding healthy tissue and providing a uniform dose.
Due to their internal shielding and small footprint, TomoTherapy Hi-Art and TomoTherapy TomoHD treatment machines are the only high energy radiotherapy treatment machines used in relocatable radiotherapy treatment suites. Two different types of suites are available: TomoMobile developed by TomoTherapy Inc. which is a moveable truck and Pioneer, developed by UK-based Oncology Systems Limited. The latter was developed to meet the requirements of UK and European transport law requirements and is a contained unit that is placed on a concrete pad, delivering radiotherapy treatments in less than five weeks.
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