3. Beam Delivery and Properties

With the exception of the first ion beam treatments using deuterium and helium beams in 1954 at Berkeley, which were cyclotron-based, only synchrotron-based therapy facilities are in operation today for carbon or other light ion beam therapy. This is due to the significantly higher energies needed to achieve the same range as a proton beam. After the early studies at the Lawrence Berkeley National Laboratory (LBNL) (Castro, 1993), all dedicated particle therapy facilities concentrated primarily on carbon beams for radiotherapy. The layout of a modern synchrotron for carbon ions is also suited to accelerate protons and many therapy facilities do offer proton beams for therapy in addition to carbon ions, which facilitates comparative clinical trials between modalities. Although technically feasible, only the Heidelberg ion beam facility (Haberer et al., 2004) is offering the possibility of providing helium and oxygen ions in addition to protons and carbon ions in all treatment rooms for potential future clinical use. The magnet strength and the fixed diameter of the accelerator ring limit the maximum achievable energy for each ion. A typical synchrotron-based ion facility comprises the following main components: (i) one or several ion sources; (ii) a linac, which acts as a pre-accelerator; (iii) the main synchrotron accelerator to produce high-energy ions; (iv) a beam-transport system to steer the beam to the treatment-delivery system; (v) a treatment-delivery system to adapt the individual treatment fields for each patient. Ion sources use an ionized gas to produce the ions, so that only specific ions can be extracted, using a spectrometer, to select a well-defined charge to mass ratio. The ionized gas is confined by magnetic fields and heated by microwaves, so that a plasma is created. Therefore, separate ion sources are in use for protons, carbon and helium ions, while an oxygen beam can be extracted from the same source as carbon (using carbon dioxide). The beam from different ion sources can be switched quickly to the injection beam line, using a switching magnet. The extraction of a different ion from the same source takes time to tune the source. The ion source is also used to control the beam intensity. The linac injector accelerates ions from several keV/u to the injection energy of the synchrotron, which is typically around 5MeV/u to 10MeV/u. In the main ring, bunches of ions are being injected, accelerated step by step to the desired energy and then extracted to the high-energy beam transport system. This means that the extracted energy can be adapted according to the energy requested by a treatment plan. During acceleration, the magnetic field strength in the ring magnets has to be gradually increased, as well as the frequency of the accelerating radio frequency (RF) cavity. The setting of all the beam elements from the ion source to the high-energy beam line is controlled by an accelerator control system (ACS). The ACS usually can provide predefined sets of ion type, energy, beam diameter, and beam intensity. In the beam delivery system (BDS), the small focused mono-energetic beam is modified, such that a useful treatment field is formed. Moreover, it is monitoring the delivered beam and has control over the delivered RBE-weighted dose to the patient. The BDS may use passive or dynamic techniques and is controlled by the treatment control system (TCS), which also has control over the ACS. The TCS also serves as an interface to the treatmentplanning system. In view of the safety of the patient, the TCS is the most important system in the facility. The BDS may furthermore comprise some or all of the following subsystems: nozzle, monitoring system, beam scanner, gating device, patient positioning and immobilization system. In practice, carbon energies of 400MeV/u (i.e., total kinetic energies of 4.8 GeV) are required to penetrate 25 cm in water (Chu et al., 1993). The higher energy of the extracted ions as compared to a proton therapy beam leads to an increased magnetic rigidity of the beam (Trbojevic et al., 2007). This is compensated for by bending magnets which have higher magnetic fields and larger bending radii. Moreover, the reduced lateral scattering and increased magnetic rigidity of high-energy ions lead to a very long BDS of typically 6m to 10m (for