DESCRIPTION OF THE EQUIPMENT CONTAINED IN DIRAC
MEGAVOLTAGE (MV) UNITS
Megavoltage (MV) units are medical devices used to deliver external beam radiotherapy (often referred to as teletherapy) to cancer patients. In DIRAC, a MV unit is either a Radionuclide Teletherapy Unit or an Electron Accelerator. In principle, the term megavoltage is conventionally used as an X-ray quality specifier for beams created with Electron Accelerators. However, the terms “megavoltage beams/photons/radiation” are commonly used to describe also the quality of Cobalt-60 radiation, no matter how the radiation is generated.
ELECTRON ACCELERATORS FOR TELETHERAPY
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 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 the electrons are injected into a linear waveguide and accelerated by the action of radio-frequency electromagnetic waves, producing the beam.
RADIONUCLIDE TELETHERAPY UNITS
Treatment devices incorporating gamma-ray emitting sources for use in external beam radiotherapy are called Radionuclide Teletherapy Units. These units use radioactive isotopes, which gives off high-energy gamma rays. Two gamma-emitting radionuclides have been widely used used in the past for 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. Modern Radionuclide Teletherapy Units can be equipped with advanced software control, collimation, in-room imaging and treatment couch. 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.
X-RAY GENERATORS FOR KILOVOLTAGE (kV) THERAPY
Treatment devices incorporating x-ray tubes to produce low energy (tube potential 40-300 kV) x-rays for use in radiotherapy are called
x-ray 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 less than 2cm (sometimes mentioned also as electronic Brachytherapy units);
- superficial units, producing x-rays for tube potentials 50-150 kV and operating at distances between 15 and 20 cm;
- orthovoltage units generating x-rays for tube potentials 150 to 300 kV and operating at distances around 50 cm.
PARTICLE ACCELERATORS FOR LIGHT ION BEAM THERAPY
A particle accelerator is a radiation treatment machine that accelerates ions using electromagnetic fields to a high energy beam.
Currently, the particles that are clinically available are protons and carbon ions.
According to a joint ICRU/IAEA recommendation, they are both light ions, i.e. ions with atomic number equal or smaller
than that of Neon (Z<=10). Their energies are in the range of around 60 - 250 MeV and 350-400 MeV/u,
respectively (where the unit MeV/u indicates the energy of each nucleon forming the ion).
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), cyclo-synchrotron or synchrotron (protons or ions) for acceleration and beam production. Finally, a high-energy beam transport system delivers a clinically useful beam. Two types of proton beam delivery systems are currently available: the broad-beam delivery system and the pencil-beam scanning (PBS) system. With the first one, the beam is spread uniformly and then conformed to the target by customized devices (i.e., collimator, range compensator). With the latter one, a narrow beam is electromagnetically scanned over the target volume in a sequence specifically designed for each target. Intensity-modulation can also be performed with PBS systems.
The peculiarity of light ions with respect to photons is a peak of energy deposition (Bragg peak) at a certain depth with a consecutive rapid fall-off. The modulation of light ion energy allows the extension of the region of maximum dose deposition. The technique is called “range modulation” and the region of high dose uniformity that is created is referred to as the “spread-out Bragg peak” (SOBP). From the therapeutic point of view, light ions have a stronger biological effect (cell killing per amount of deposited dose) than photons.
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. According to the dose rate (dose delivered per unit of time) to the point of prescription of the dose, brachytherapy is classified into three categories: Low Dose Rate (LDR) brachytherapy ranges between 0.4 and 2 Gy/h, Medium Dose Rate (MDR) brachytherapy ranges between 2 Gy/h and 12 Gy/h and High Dose Rate (HDR) brachytherapy, where dose is delivered at 12 Gy/h or more. Pulsed Dose Rate Brachytherapy (PDR) delivers the dose in a large number of small fractions with short intervals, mimicking the radiobiology of LDR brachytherapy.
Brachytherapy sources are usually inserted (loaded) into catheters or applicators. Sources can be hot-loaded (the applicator is preloaded with LDR radioactive sources at the time of placement into the patient - manual brachytherapy) 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 either Cobalt-60 or Iridium-192 gamma-ray sources or miniaturized high dose rate x-ray generators, with a computer-controlled source-drive mechanism for use in remote afterloading HDR brachytherapy, are called afterloaders or afterloader units. Palladium-103, Iodine-125, Cesium-137, Ytterbium-169 and Iridium-192 are some of the photon-emitting LDR isotopes. Ocular LDR applications are in some centres performed with beta-emitting plaques made of shielded Ruthenium/Rhodium-106. In DIRAC, data of both afterloading units and LDR installations (i.e. services performing LDR applications) are collected.
The CT-simulator is a dedicated CT scanner for use in radiotherapy treatment simulation and planning. CT-simulators require a large bore (up to 85 cm in diameter) to allocate the room for patient fixation tools. A flat tabletop is available for patient positioning and marking, to mimic the conditions of radiotherapy treatment delivery. Additionally, CT-simulators are usually equipped with movable room lasers and a special software for virtual simulation.
The (conventional) simulator is a machine that emulates the geometry and movements of the treatment unit with a diagnostic quality x-ray source in place of the megavoltage x-ray source. 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 personalised characteristics of the radiotherapy technique for a patient. 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.