[0:01]How the Linear Accelerator works. All 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 kill the abnormal cancer cells in a tumor. Successful radiation therapy depends on the ability of the linear accelerator or Linac to deliver a tumorcidal dose of radiation to the tumor. While ensuring minimal radiation of normal tissue, the Linac accurately produces, monitors, controls and conforms the radiation beam to the planned target. Beam generation. Radio frequency waves are pulsed into the wave guide by the magnetron. This is synchronized with the injection of electrons into the wave guide by the electron gun. The radio frequency waves accelerate the electrons along the wave guide to a 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 radio frequency 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 wave guide. The electrons are produced by heating the tungsten filament within the cathode and are then injected into the wave guide. The number of electrons injected is controlled by the temperature of the filament. The electrons are accelerated along the wave guide toward the target. The wave guide contains a series of copper cells. Small holes or irises between these copper cells allow the electrons to travel along the wave guide 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 two sets of quadrupole magnets, called steering coils, that surround the wave guide. An additional two 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 pin head when it hits the target. The entire system is cooled by water. The electrons exit the wave guide and enter the flight tube, where the beam is redirected towards the target. The electrons travel along a slalom path within the flight tube. Three pairs of magnets on either side of the flight 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 it also further focuses the beam to a diameter of 1 mm. 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 bending is unique to Electra linear accelerators. It helps to minimize the size of the machine and ensures that its center remains low, which is important for patient 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 forward traveling x-rays to pass through, creating a cone-shaped beam. The primary collimator minimizes leakage and therefore excess total body dose by absorbing scattered x-rays traveling in the lateral direction. It also defines the maximum size of the resulting clinical radiation beam.
[4:24]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 than from the sides, creating a uniform photon beam. Dose measurement. The photons now pass through the ion chamber for dose measuring and beam quality monitoring. The dose delivered to the patient is measured and controlled simultaneously in two independent ionization chambers. One chamber is the primary dosimeter. 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 irradiation if the primary chamber fails. The treatment machine must replicate the beams modeled within the planning system. This is critical to the accuracy of treatment delivery. A beam quality function is performed by a third ionization chamber, which uses seven electrodes to monitor different sections of the radiation field. The x-ray beam is almost ready to treat the patient. Before that, further 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, a number of fine tungsten leaves which move independently of one another and can create a variety of complex treatment shapes.
[6:17]Linac control. One computer system controls both the Linac and the multi-leaf collimator. This eliminates dosimetric 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 electromagnet, steering and focusing coils are digitally controlled. All mechanical positions of flight tube, filters and foils are automatically selected from the control console, and all radiation beam settings are grouped in calibration blocks for each energy. These are stored digitally on the Linac's hard disk for flexible and easy beam adjustment, calibration and servicing. Clearance. Clearance is 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 isocenter, 45 cm, and the head diameter, 62 cm. A wide clearance around the isocenter means improved access for patient setup. Freedom to use the best possible patient positioning and immobilization accessories. Freedom to rotate the gantry between fields without needing to move the patient. Finally, it means that treatment techniques using non-coplanar beams are not compromised. The large clearance offered by Electa machines ensures flexibility in providing the best possible treatment for the patient.
