Tomotherapy

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Tomotherapy
Tomotherapy.jpg
Tomotherapy Hi Art machine
Other namesHelical tomotherapy
Specialty oncology

Tomotherapy is a radiation therapy modality, [1] [2] [3] in which the patient is scanned across a modulated strip-beam, so that only one “slice” (Greek prefix “tomo-”) of the target is exposed at any one time by the linear accelerator (linac) beam. The three components distinctive to this modality are: (1) a collimator pair that defines the length of the strip, (2) a binary multileaf collimator whose leaves open and close during treatment to modulate the strip’s intensity, and (3) a couch that scans the patient across the beam at a fixed speed the during treatment delivery.

Contents

General principles

The treatment field’s length is selectable. In static-jaw delivery, the field length remains constant during a treatment. In dynamic-jaw delivery, the field length changes so that it begins and ends at its minimum setting.

Patient undergoing tomotherapy, face and body covered, to prevent movement. Tomotherapy nci-vol-4478-300.jpg
Patient undergoing tomotherapy, face and body covered, to prevent movement.

TomoTherapy treatment times vary compared to normal radiation therapy treatment times (tomotherapy treatment times can be as low as 6.5 minutes for common prostate treatment [4] ) but do add an additional 23 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).

There are few head to head comparisons of tomotherapy and other IMRT techniques, however there is some evidence that VMAT can provide faster treatment while tomotherapy is better able to spare surrounding healthy tissue while delivering a uniform dose. [5] [6] [7]

Helical Delivery

In helical tomotherapy, the linac rotates on its gantry at a constant speed while the beam is delivered; so that from the patient’s perspective, the shape traced out by the linac is helical.

While helical tomotherapy can treat very long volumes without a need to abut fields in the longitudinal direction, it does display a distinct artifact due to "thread effect" [8] when treating non-central tumors. Thread effect can be suppressed during planning through good pitch selection.

Fixed-Angle Delivery

Fixed-angle tomotherapy uses multiple tomotherapy beams, each delivered from a separate fixed gantry angle, in which only the couch moves during beam delivery. This is branded as TomoDirect, but has also been called topotherapy. [9]

The technology enables fixed beam treatments by moving the patient through the machine bore while maintaining specified beam angles.

Clinical Considerations

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. [10] [11] [12]

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. [13]

Some research has suggested tomotherapy provides more conformal treatment plans and decreased acute toxicity. [14]

Non-helical static beam techniques such as IMRT and TomoDirect are well suited to whole breast radiation therapy. These treatment modes avoid the low-dose integral splay and long treatment times associated with helical approaches by confining dose delivery to tangential angles. [15] [16] [17]

This risk is accentuated in younger patients with early-stage breast cancer, where cure rates are high and life expectancy is substantial. [17]

Static beam angle approaches aim to maximize the therapeutic ratio by ensuring that the tumor control probability (TCP) significantly outweighs the associated normal tissue complication probability (NTCP). [18] [19] [20]

History

The tomotherapy technique was developed in the early 1990s at the University of Wisconsin–Madison by Professor Thomas Rockwell Mackie and Paul Reckwerdt. [21] 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). [13]

The first implementation of tomotherapy was the Corvus system developed by Nomos Corporation, with the first patient treated in April, 1994. [22] [10] 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 [23] eschewed the fixation system and applied the technique to tumors in many different parts of the body.

At this time, the systems manufactured by Accuray (previously TomoTherapy Inc.) are the primary tomotherapy devices in use.

Mobile tomotherapy

Due to their internal shielding and small footprint, TomoTherapy Hi-Art and TomoTherapy TomoHD treatment machines were the only high energy radiotherapy treatment machines used in relocatable radiotherapy treatment suites. Two different types of suites were available: TomoMobile developed by TomoTherapy Inc. which was 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 was a contained unit placed on a concrete pad, delivering radiotherapy treatments in less than five weeks. [24] [25]

See also

Related Research Articles

Radiation therapy Therapy using ionizing radiation, usually to treat cancer

Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radiation therapy may be curative in a number of types of cancer if they are localized to one area of the body. It may also be used as part of adjuvant therapy, to prevent tumor recurrence after surgery to remove a primary malignant tumor. Radiation therapy is synergistic with chemotherapy, and has been used before, during, and after chemotherapy in susceptible cancers. The subspecialty of oncology concerned with radiotherapy is called radiation oncologist.

External beam radiotherapy Treatment of cancer with ionized radiation

External beam radiotherapy (EBRT) is the most common form of radiotherapy. The patient sits or lies on a couch and an external source of ionizing radiation is pointed at a particular part of the body. In contrast to brachytherapy and unsealed source radiotherapy, in which the radiation source is inside the body, external beam radiotherapy directs the radiation at the tumour from outside the body. Orthovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures. Megavoltage X-rays are used to treat deep-seated tumours, whereas megavoltage electron beams are typically used to treat superficial lesions extending to a depth of approximately 5 cm. X-rays and electron beams are by far the most widely used sources for external beam radiotherapy. A small number of centers operate experimental and pilot programs employing beams of heavier particles, particularly protons, owing to the rapid dropoff in absorbed dose beneath the depth of the target.

A radiation oncologist is a specialist physician who uses ionizing radiation in the treatment of cancer. Radiation oncology is one of the three primary specialties, the other two being surgical and medical oncology, involved in the treatment of cancer. Radiation can be given as a curative modality, either alone or in combination with surgery and/or chemotherapy. It may also be used palliatively, to relieve symptoms in patients with incurable cancers. A radiation oncologist may also use radiation to treat some benign diseases, including benign tumors. In some countries, radiotherapy and chemotherapy are controlled by a single oncologist who is a "clinical oncologist". Radiation oncologists work closely with other physicians such as surgical oncologists, interventional radiologists, internal medicine subspecialists, and medical oncologists, as well as medical physicists and technicians as part of the multi-disciplinary cancer team. Radiation oncologists undergo four years of oncology-specific training whereas oncologists who deliver chemotherapy have two years of additional training in cancer care during fellowship after internal medicine residency in the United States.

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Radiosurgery

Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy, it is usually used to treat cancer. Radiosurgery was originally defined by the Swedish neurosurgeon Lars Leksell as "a single high dose fraction of radiation, stereotactically directed to an intracranial region of interest".

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Fast neutron therapy

Fast neutron therapy utilizes high energy neutrons typically between 50 and 70 MeV to treat cancer. Most fast neutron therapy beams are produced by reactors, cyclotrons (d+Be) and linear accelerators. Neutron therapy is currently available in Germany, Russia, South Africa and the United States. In the United States, three treatment centers are operational in Seattle, Washington, Detroit, Michigan and Batavia, Illinois. The Detroit and Seattle centers use a cyclotron which produces a proton beam impinging upon a beryllium target; the Batavia center at Fermilab uses a proton linear accelerator.

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