DESCRIPTION OF THE EQUIPMENT CONTAINED IN DIRAC
Electron accelerators are radiation treatment devices that accelerate electrons using electromagnetic fields to a required energy (from 4 MeV for a low energy machine to a few tens of MeV for a higher-energy machine), useful for medical purposes. Strictly speaking, the "accelerator" is only the part of the machine in which electrons are accelerated, but the term "electron accelerator" or "clinical accelerator" is used to describe the whole machine used to deliver radiotherapy. Electrons are produced by thermionic emission (emission of electrons from a hot cathode into a vacuum tube) in the electron gun. These electrons are then injected into the electron accelerator and the generated electron beam is guided to the treatment head, where it is subsequently modulated and prepared for medical use. X-ray photons are produced when these accelerated electrons collide with a metallic target (a thin layer of a high-Z material such as tungsten). There are two basic types of electron accelerators according to the acceleration method: circular accelerators (betatron, microtron) and linear accelerators (often shortened to LINAC). In circular accelerators, electrons generated in the gun are injected into a torus-shaped vacuum tube placed into the gap between two magnet poles. The electro-magnetic field accelerates the electrons and generates the beam. In a linear accelerator electrons are injected into a linear waveguide and accelerated by the action of radio-frequency electromagnetic waves, producing the beam.
RADIONUCLIDE TELETHERAPY UNIT
Treatment devices incorporating gamma-ray emitting sources for use in external beam radiotherapy are called teletherapy machines. Two gamma-emitting radionuclides have been used in external beam teletherapy: cobalt-60 and cesium-137. The use of cesium-137 for external beam radiotherapy was discontinued during the 1980s, due to the problems associated to the low activity (large source size and short treatment distance). Cobalt-60 has been the most widely used isotope for teletherapy, because it offers a good compromise between the energy of emitted photons, half-life, specific activity, and means of production. The source movement from beam-on to beam-off (storage) position is done with mechanical or pneumatic methods. A highly sophisticated cobalt-60 treatment unit dedicated to stereotactic radiosurgery uses an array of separate cobalt sources housed in the central body of the unit producing collimated beams directed to a single focal point.
Treatment devices incorporating x-ray tubes to produce low energy (tube potential 40-300 kV) x-rays for use in external beam radiotherapy or brachytherapy are called x-rays generators. The cathode of the x-ray tube expels the electrons from the electrical circuit by thermionic emission. The electrons are accelerated electrostatically and strike the anode (composed of a high-Z material such as tungsten), where the kinetic energy of the electrons is transformed into x-rays (both bremsstrahlung and characteristic x-rays). According to the energy of the generated x-rays there are three types of x-rays generators: contact units producing typically x-rays for tube potentials 40-50 kV, operating at distances <2cm (electronic brachytherapy falls in this category), superficial units, producing x-rays for tube potentials 50-150 kV and operating at distances between 15 and 20 cm, and orthovoltage units generating x-rays for tube potentials 150 to 300 kV and operating at distances around 50 cm.
A particle accelerator is a radiation treatment device machine that accelerates hadrons (heavy particles such as protons and carbon ions) using electromagnetic fields to high energy beams (60 - 250 MeV for protons and 350-400 MeV for carbon ions), useful for medical purposes. The dose deposition of high energy particles is much like that of photons originated in an electron accelerator. The therapeutic difference is the stronger biological effect (cell killing per amount of deposited dose) of particles. Particle accelerators use electric fields to increase the speed and the energy of a beam of particles, which are guided and focused by magnetic fields. Unlike electron accelerators, that are compact devices by design, particle accelerators require a complex set-up of different elements, which require significant space. An ion source provides the particles, such as protons or ions, which are to be accelerated. The positively charged particles are formed from electron bombardment of a gas and extracted from the resulting plasma. The injector transports the particles into a vacuum chamber to a cyclotron (protons) or synchrotron (protons or ions) for acceleration and beam production. Finally, a high-energy beam transport system delivers a clinically useful beam.
Brachytherapy concerns primarily the use of radioactive sealed sources (or miniaturized high dose rate x-ray generators in the case of electronic brachytherapy) placed directly into tissue either inside or very close to the target volume. Brachytherapy sources are usually inserted (loaded) into catheters or applicators. Sources can be hot-loaded (the applicator is preloaded with radioactive sources at the time of placement into the patient) or after-loaded (the applicator is placed first and the radioactive sources are loaded later, either by hand in case of manual afterloading or by a machine in the case of automatic remote afterloading). Treatment devices incorporating gamma-ray sources or miniaturized high dose rate x-ray generators with a computer controlled source-drive mechanism for use in brachytherapy are called afterloaders or afterload units. Both cobalt-60 and iridium-192 isotopes are used in modern afterloaders. According to the dose rate (dose delivered per unit of time), brachytherapy is classified into three categories: Low Dose Rate (LDR) brachytherapy ranges between 0.4 and 2 Gy/h (this is compatible with manual or automatic afterloading techniques), Medium Dose Rate (MDR) brachytherapy ranges between 2 Gy/h and 12 Gy/h (also compatible with manual or, more frequently, automatic afterloading) and High Dose Rate (HDR) brachytherapy, where dose is delivered at 12 Gy/h or more (only compatible with automatic afterloading because of the high source activity). Pulsed Dose Rate Brachytherapy (PDR) delivers the dose in a large number of small fractions with short intervals, mimicking the radiobiology of LDR brachytherapy.
The CT-simulator is a dedicated CT scanner for use in radiotherapy treatment simulation and planning. CT-simulators usually have large bore (opening up to 85 cm) and are equipped with room lasers, including a movable sagittal laser for patient positioning and marking and flat table top and special software for virtual simulation.
The (conventional) simulator is a machine that emulates the geometry and the movements of the treatment unit with a diagnostic quality x-ray source in place of the megavoltage x-ray source of the treatment unit. Planar x-ray reference images of the patient in the treatment position are generated with an image intensifier or flat panel detector, or with radiographic film.
TREATMENT PLANNING SYSTEM
Treatment planning is a process in which the care team defines the characteristics of the radiotherapy technique for a patient with cancer. Treatment planning consists of many steps including adequate patient diagnosis and staging, image acquisition for treatment planning, the localization of the volumes of interest, beam arrangement and its optimization, and treatment simulation. The treatment planning system (TPS) is the combination of software and hardware used to generate the treatment beam geometry and calculate the expected dose distribution in the patient’s tissue. Many configurations of software (dose calculation algorithms) and hardware are possible, making the TPS highly configurable equipment.