Conventional Linear Accelerators

By Author: Melva Lynch
Total Articles: 37
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Radiotherapy works by targeting high-energy gamma ray or 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.

The contemporary linear accelerators provide either 6 MeV photons in the "low energy" range machines, or dual/triple photon energy and several electron energies.

The following systems are incorporated into the linear accelerators: multileaf collimator (MLC) for beam shaping, capability to deliver intensity modulated radiotherapy (IMRT) and a 3D image guidance system (IGRT). The linear accelerator also provides an electronic portal imaging system for 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 linear accelerators, their high patient throughput 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 regulations, guidelines for radio therapy room design, etc.

Technical Considerations

The technology used in conventional linear accelerators is essentially the same for all linear accelerators. The high energy generation can be achieved either by implementing the "traveling waveguide technology" or by implementing the "standing waveguide technology". The first approach is simpler and very reliable, while the second approach 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 tumor. The wave guide, the filters and the collimator are mounted on a gantry which rotates around the patient, allowing the tumor to be irradiated from multiple directions. There is a linear accelerators patient coach especially designed to allow irradiation of the patient from multiple directions.

Modern linear accelerators are designed to enable intensity- modulated radiotherapy (IMRT). This is a dynamic radiotherapy delivery method which enables good control over the three-dimensional dose distribution: the delivered beam instead of being flat, changes 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 field.

* 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 leaves are all moving continuously. 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 detector used for the portal imaging consists of an amorphous silicon indirect detection flat panel imager.

The imager 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-ray cone beam, in plain or orthogonal to the linear accelerators' high energy 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 skin 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 allow a wide functionality, including booking and scheduling, recording of patient notes, recording costs, image handling etc.

Linear Accelerators: 20/18

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