Radiotherapy works by targeting high-energy ray electron beam onto cancer cells, which are more prone to radiation damage than the normal tissues. Linear accelerators are usually multi-modality ionizing radiation generators with associated imaging devices, currently used for delivery of external beam radiotherapy.
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The contemporary linear accelerator provides either 6 MeV photons in the “little energy” range machines, or dual/triple photon energy and several electron energies.
The following systems are incorporated into the linear accelerators: multileaf col-limator (MLC) for beam shaping, the capability to deliver intensity modulated radio-therapy (IMRT) and a 3D imaging treatment beams.
Other additional features may be incorporated into the linear accelerators, such as arc-based IMRT delivery, total body irradiation, high-dose rate electrons, etc.
Despite the high capital cost of purchasing them, their high patient through put over a long lifespan makes them extremely cost-effective compared with the other treatment options.
The operation of linear accelerators should be according to the existing standards and regulations which include: IEC standards, radiation, guidelines for radio therapy room design, ETC.
Technical Considerations:
The technology used in conventional linear accelerators is essentially the same for all linear accelerates. The high-energy generation can be achieved either by implementing the “travelling waveguide technology” or by applying the “standing waveguide technology”. The first approach is simpler and very reliable, while the second method features a more stable treatment beam. The generated radiation beam is flattened and shaped using collimation devices to form a beam matching the shape of the tumour. The wave guide, the filters and the collimator are mounted on a gantry which rotates around the patient, allowing the tumour to be irradiated from multiple directions. There is a linear accelerator patient coach primarily designed to allow irradiation of the patient from multiple addresses.
Modern linear accelerators are designed to enable intensity – modulated radiotherapy (IMRT). This is a dynamic radiation delivery method which allows real control over the three-dimensional dose distribution: the delivered beam instead of being flat changed the intensity at different points within the beam.
There are different types of linear accelerator IMRT delivery methods:
Step and shoot method: this method consists of delivering a series of different shape fields formed by the MLC to build up a variable intensity pattern. The irradiation is stopped between each area.
Dynamic MLC methods: in which the radiation runs constant, while the MLC leaves move across the field at variable speeds.
Arc therapy methods: in which the radiation runs continuously, and the gantry collimators and MLC leavers are all running (always). The dose rate also varies during the delivery. Treatments are delivered in an arc (or sometimes in two arcs). This is probably the best choice for IMRT.
The multileaf collimator is a delicate computer-controlled mechanism. The performance of the collimator depends on its resolution, the leakage radiation and the system penumbra.
The electronic portal imaging device allows verification of patient position on orthogonal two-dimensional images produced by the high-energy treatment beam. The detection used for the portal imaging consists of an amorphous silicon indirect detection flat panel imager.
The image systems differ in the degree of complexity of software provided, as well as the ease of use.
The linear accelerators three-dimensional imaging systems can be materialized in three different ways:
1. Applying diagnostic X-rat cone beam, in plain or orthogonal to the linear accelerators high enery beam.
2. Using the high-energy treatment cone beam of the linear accelerator
3. Using a CT machine combined with the linear accelerator.
Photons and Electron Energies
Typically, X-ray energies of 6 MeV are used for head and neck, breast and lung radiation. Abdominal treatments usually utilize 10 MeV X-ray. Large patients may require higher X-ray energies.
Electrons are usually required in the 6-15 MeV range. The total electron treatments require 4 MeV electrons.
Record and Verify systems
In modern radiotherapy departments, the linear accelerators are operated using record and verify systems, which deliver the treatments, as well as make a record of them.
These systems are usually connected to the general information network, which allows an extended functionality, including booking and scheduling, recoding of patients notes, recording costs, image handling, etc.
Acceletronics is an independent service company dedicated to delivering the best equipment performance and service reliability from linear accelerators and CT scanners across all major breands and models. Learn more about acceletronics and their selection of new and refurbished linear accelerators and CT scanners today at www.acceletronics.com. To Contact one of our LINAC experts call 610-524-3300