Nowadays in cancer treatment radiotherapy (RT) using Linear Accelerator (Linac machine) becoming very popular.
Because of the good success rate of recovering from cancer using radiotherapy or including with chemotherapy.
The concept of the machine is similar to cyclotron and particle smasher. But more sophisticated and controlled due to dose control and accuracy for the radiotherapy treatment.
The body cells can be damaged or killed by radiation but tumor cells are more sensitive to radiation than normal cells.
Radiation therapy uses this principle to damage beyond repair or kills the abnormal cancer cells in a tumor.
Successful radiation therapy depends on the ability of the linear accelerator (Linac machine) to deliver to a more subtle dose of radiation to the cancer tissues while ensuring minimal radiation of normal tissues.
Medical Linear Accelerator ( Linac machine )
How does the linear accelerator work? The Linac machine accurately produces monitors controls and conforms the radiation beam to the planned target.
The new generation radio frequency wave pulses are feed into the waveguide by the magnetron.
This is synchronized with the injection of electrons into the waveguide by the electron gun. The radiofrequency waves accelerate the electrons along the waveguide to speed approaching the speed of light.
The x-ray beam is created when the electrons hit and interact with a tungsten target at the opposite end.
The magnetron controls the power and frequency of the radiofrequency waves which determine the energy of the x-rays produced.
The digital accelerator uses a diode type electron gun situated at the end of the waveguide the electrons are produced by heating the tungsten filament within the cathode and then injected into the waveguide.
The number of electrons injected is controlled by the temperature of the filament. The electrons are accelerated along the waveguide toward the target the waveguide contains a series of copper cells.
Small holes or irises between these copper sales allow the electrons to travel along the waveguide and help to focus the beam. A vacuum is created to ensure that the electron beam is not impeded by other particles.
The path of the negatively charged electron beam is controlled by 2 sets of quadrupole magnets called steering coils that surround the waveguide.
An additional 2 sets of focusing coils help to further define the electron beam so that it is very fine with a diameter similar to that of a pinhead when it hits the target.
The entire system is cooled by water. The electrons exit the waveguide and enter the flat tube where the beam is redirected towards the target.
The electrons travel along the path within the flat tube. 3 pairs of magnets on either side of the flat tube cause the electron beam to bend through the turns of the slalom.
This process not only positions the beam to strike the target but also further focuses the beam to a diameter of one millimeter.
The design of the magnets enables them to focus electrons of slightly different energies onto the same point on the target.
This is called a chromatic behavior this slalom winding is unique to elect the linear accelerators.
It helps to minimize the size of the machine and ensures that its ISO center remains low which is important for patients setup.
The high-energy electrons hit a small tungsten target where the electron energy is converted into photons or x-rays.
The high-energy photons emerge from the target in a variety of directions the primary collimator only allows for traveling x-rays to pass through creating a cone-shaped beam.
The primary collimator minimizes leakage and therefore access the total body does by absorbing scattered x-rays traveling in the lateral direction.
It also defines the maximum size of the resulting clinical radiation beam.
At this stage the photons are not uniformly distributed across the beam and so a flattening filter is placed in the path of the beam.
The cone-shaped filter absorbs more photons from the center of the beam down from the sides creating a uniform photon beam.
Dose measurement
The photons now passed through the ionization chamber for dose-measuring and beam quality monitoring.
The dose delivered to the patient is measured and controlled simultaneously into independent ionization chambers.
One chamber is the primary to the senator. It measures the radiation and terminates the beam when the required dose has been delivered.
The secondary ion chamber acts as a backup and will stop the radiation if the primary chamber fails.
The treatment Linac machine must replicate the beams modeled within the planning system. This is critical to the accuracy of treatment delivery.
Beam quality function is performed by a third ionization chamber which uses 7 electrodes to monitor different sections of the radiation field.
The x ray beam is almost ready to treat the patient. Before that, though the beam shaping is required to ensure that the shape of the delivered x ray beam matches the shape of the tumor.
This is done using a multi-leaf collimator the number of fine tungsten leaves which move independently of one another and can create a variety of complex treatment shapes.
Control. One computer system controls both the linac and the multi-leaf collimator this eliminates dosimetry errors due to communication delays.
It also ensures synchronization between the delivered dose and the multi-leaf collimator position allowing complex deliveries such as intensity-modulated radiation therapy and volumetric modulated arc therapy.
All electromagnets steering and focusing coils are digitally controlled on the mechanical positions of flight 2 filters and foils are automatically selected from the control console and radiation beam settings are grouped in calibration blocks for each energy.
These are stored digitally on the Lynette hard disk for flexibility and ease being adjustment calibration and servicing.
Clearances the free space available under the linac for patient treatment and it varies with different protocols and fixation devices.
It is a combination of the distance between the lower surface of the radiation head and the ISO center 45 centimeters and the head diameter 62 centimeters.
Wide clearance around the ISO center means. Access for patients set up. The freedom to use the best possible patient positioning and immobilization accessories.
Freedom to rotate the gantry between feels without needing to move the patient finally means that treatment techniques using non-coplanar beams are not compromised.
The large clearance offered by elected machines ensures flexibility in providing the best possible treatment for the patient.
To complete study about X-Ray fundamentals and it’s application follow X-Ray.
Components of modern Linac machine
Linacs machines are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine.
1 Gantry
2 Gantry stand or support
3 Modulator cabinet
4 Patient support assembly (table)
5 Control console
However, there are significant variations from one commercial machine to another, depending on the final electron beam kinetic energy as well as on the particular design used by the manufacturer.
The length of the accelerating waveguide depends on the final electron kinetic energy and ranges from ~30 cm at 4 MeV to ~150 cm at 25 MeV.
The main beam forming components of a modern medical linac are usually grouped into six classes.
1 Injection system
2 RF power generation system
3 Accelerating waveguide
4 Auxiliary system
5 Beam transport system
6 Beam collimation and beam monitoring system
RT planning for Linac machine
CT simulators are CT scanners equipped with special features that make them useful for certain stages in the radiotherapeutic process.
The special features are:
1) Flat table top surface to provide a patient position during simulation that will be identical to the position during treatment on a megavoltage machine.
2) Laser marking system to transfer the coordinates of the tumour isocentre, derived from the contouring of the CT data set, to the surface of the patient.
Two types of laser marking systems are used:
A) gantry mounted laser
B) system consisting of a wall mounted moveable sagittal laser and two stationary lateral lasers.
3) Virtual simulator consisting of software packages that allow the user to define and calculate a treatment isocentre and then simulate a treatment using digitally reconstructed radiographs (DRRs).
4) CT simulator essentially obviates the need for conventional simulation by carrying out two distinct functions:
A) Physical simulation, which covers the first three of the six target localization steps listed above;
B) Virtual simulation, which covers the last three of the six target localization steps listed above.
In CT simulation the patient data set is collected and target localization is carried out using CT images with fluoroscopy and radiography replaced by DRRs.
Laser alignment system is used for marking and a virtual simulator software package is used for field design and production of verification images.
Transfer of all necessary information to the TPS is achieved electronically.
The planar simulation X ray film provides a beam’s eye view (BEV) of the treatment portal but does not provide 3-D information about anatomical structures.
On the other hand, CT provides anatomical information and target definition but does not allow a direct correlation with the treatment portals.
DRR is the digital equivalent of a planar simulation X ray film.
It is reconstructed from a CT data set using virtual simulation software available on a CT simulator or a TPS and represents a computed radiograph of a virtual patient generated from a CT data set representing the actual patient.
Just like a conventional radiograph, the DRR accounts for the divergence of the beam. The basic approach to producing a DRR involves several steps like:
1) Choice of virtual source position.
2) Definition of image plane.
3) Ray tracing from virtual source to image plane.
4) Determination of the CT value for each volume element traversed by the ray line to generate an effective transmission value at each pixel on the image plane.
5) Summation of CT values along the ray line (line integration).
6) Grey scale mapping.
An extension of the DRR approach is the digitally composited radiograph (DCR), which provides an enhanced visualization of bony landmarks and soft tissue structures.
This is achieved by differentially weighting ranges of CT numbers that correspond to different tissues to be enhanced or suppressed in the resulting DCR images.
To know about ct, how it works or reconstruct body internal organ images using x-ray follow ct scan.
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