1. RADIOTHERAPY WITH CARBON ION BEAMS
1.4 B EAM DELIVERY SYSTEMS AND ACCELERATORS FOR HADRONTHERAPY
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1.4 Beam delivery systems and
accelerators for hadrontherapy
In order to fully to take the previously discussed physical and biological ad-vantages of heavy ions, particular requirements with stringent specifications have to be satisfied by the dose delivery systems and, sometimess, complex technologies have to be developed to deliver the optimum radiation dose distri-butions, obtaining the tumour cells inactivation and at the same time the sparing of surrounding healthy tissues. An hadrontherapy centre should meet all these requirements which can be briefly summarized as follows [3]:
- ability to treat most cancers at any depth;
- treatment times as short as possible;
- high ability to discriminate between the volume to be treated and the sur-rounding - healthy tissues;
- availability of treatment rooms with access for beam fixed and mobile lines;
- great accuracy (≤ 3%) in absolute and relative dosimetry;
It is therefore necessary to define a beam of hadrons in terms of path, modu-lation of the Bragg peak, adjustment of the depth of penetration (range), dose rate, field size, uniformity and symmetry of the field, penumbra side and dose distal fall off.
The minimum range required for a line dedicated to the treatment of deep seated tumours is 22 cm in water. The range of a beam a 250 MeV proton beam is approximately 38 cm in water. The equivalent energy demanded to a carbon ion beam to reach acceptable penetration depths is equal to 400 AMeV, which means about 28 cm in water.
1.4.1 Beam delivery systems
Ion beams are accelerated to the required energies by acceleration machines.
Then the beam is transported through an energy selection system, which consists of a variable thickness degrader, used to modulate the energy of the beam. It is mandatory in case fixed-extraction energy accelerators are used in the centre.
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Beam lines provide the required focussing and achromatic bends to deliver a round beam with the appropriate trajectory for transmission through the gan-tries5 or the fixed beam lines. The two available options for beam delivery sys-tems are: passive scattering syssys-tems and active scanning syssys-tems.
Figure 1.17. Scheme of a passive beam shaping system. Top: lateral beam profile changes according to the elements traversed. Bottom: depth dose distribution before and after the passage through range-modulator and rang-shifter. In the middle: schematic representation of the elements. In general, the passive systems have two tasks: lateral scattering of the beam across the tumour volume and depth modulation.
An example of passive system is showed in Figure 1.17. In the passive scat-tering system a scatterer is used to expand the beam, followed by a collimator which cuts far-axis particles. A fine degrader adjusts continuously the beam en-ergy according to the tumour depth. A ridge filter is usually used as a range modulator: it expands the energy spread of the beam to the extent which corre-sponds to the tumour thickness. Thus, it is possible to achieve an appropriate SOBP for each different tumour. A bolus, which is a sort of absorber compensa-tor, conforms the dose distribution in the longitudinal direction and a final
5 Particular devices used in order to provide variable incidence angles between the hori-zontal plane and the beam; it is often considered as a major component of a facility for its complexity and cost.
1.4 Beam delivery systems and accelerators for hadrontherapy
47 mator in the lateral direction, following the tumour configuration [69].
In the active scanning system the lateral adjustment to the tumour volume is done by intensity controlled magnetic scanning systems, which allow the irradi-ated volume to be conformed to the planned target volume within a millimetre precision. An example of active system is represented in Figure 1.18. Each layer of equal particle energy is covered by a grid of individual picture elements (pix-els), for which the individual number of particles has been calculated before, to achieve the desired dose distribution [70]. The beam is moved from one pixel to the next one using a pair of magnets perpendicular to each other and to the beam direction and driven by fast power supplies. The particle fluence for each pixel is measured and the beam is switched to the next pixel when the intensity for one position has been reached. The active shaping system usually is coupled to the direct energy modulation by mean of the accelerator, which can be available, for example, using synchrotrons. Two main approaches are used for magnetic scan-ning: the spot scanning, in which the intensity of beam is measured for each voxel and the beam is turned off during the change of position, and the raster scanning, in which pixels are much closer and the beam is moved without inter-ruption, almost continuously in case of high beam intensities.
Figure 1.18. Scheme of an active scanning system. The tumour is dissected in slice and each isoenergy slice is covering by a grid of pixels for which the number of particles has been calculated before. During the irradiation the beam is guided by the magnetic sys-tem.
Passive systems represent the easiest methods when a fast energy variation
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from the accelerator is not possible. The disadvantages of this method are: a higher amount of projectile fragments, which contributes to a mixed radiation field and an enhanced lateral scattering, mainly due the protons and other lighter fragments. Moreover the high RBE part cannot be completely restricted to the tumour volume [44]. Active systems are much more precise, so that a tumour adjacent to critical organs like brain stem or optical nerves can be covered with a high dose and the normal tissue is spared to a maximum, but they require a ma-jor technological complexity [25].
Till 1997 relatively simple passive spreading systems have been used in overall the centres, most of them based on modified nuclear physics accelerators.
Only in 1997 at PSI (Villigen - Switzerland) a novel active spreading system (in-tegrated in a compact gantry) has been implemented [71].In the same years a dif-ferent active spreading system was installed in the carbon ion beam of GSI (Darmstadt).
The required characteristic for the ion beams used in the treatment together with the suitable beam delivery system, have a decisive influence in the choice of accelerator to use. It is a widespread opinion that the particle accelerator is the key element of a hadrontherapy facility because defines unequivocally the over-all technological performances, having a great impact on the quality of the radia-tion treatments produced.
1.4.2 Accelerators in hadrontherapy
The three main options for the accelerator designed specifically to operate in the hospital settings are the cyclotron, the synchrotron and the linear accelera-tor, and sometimes also a combination of them. Considering the specific re-quirements which a hadrontherapy centre have to fulfil, linear accelerators do not represent the appropriate choice because of the large space required and their costs of maintenance, therefore, they are not used in hadrontherapy. On the con-trary circular machines permit to exploit a unique accelerating structure many times, so that a more and more increased velocity of the accelerated particles is achieved in compact configurations. That is one of the reasons why cyclotrons and synchrotrons have been chosen from the beginning as “election” machines
1.4 Beam delivery systems and accelerators for hadrontherapy
49 for external radiotherapy with hadrons.
In circular accelerators particles must meet for each acceleration cycle an electric field which has to be concordant with their motion direction (synchro-nism conditions). Cyclotrons are very compact machines with a constant mag-netic field and a fixed frequency of accelerating voltage. The accelerated parti-cles, usually injected in the central part of the machine, move in a nearly spiral trajectory, increasing their energy and orbit radius. All particles travel at the same revolution frequency in the accelerator regardless of their energy and orbit radius (isochronous cyclotron). Thus, acceleration can occur continually and al-low continuous extraction of the beam. They are fixed-energy accelerator ma-chines capable of very high current. Moreover its beam structure is so that high intensities can be achieved essentially in a continuous regime. Synchrotrons are pulsed accelerators, with particles moving on a closed, approximately circular trajectory where the magnetic field and frequency of the accelerating voltage vary in time as the energy of particles increases. The energy of the extracted beam can be changed on a pulse-pulse basis, therefore allowing a direct modula-tion [3].
A great scientific debate is underway for decades on the choice between cy-clotrons and synchrotrons, in light of the strengths or weaknesses of each of the two solutions. Cyclotrons are usually appreciated for the simplicity of design and their easy of use. The main advantages are related to the structure of the ex-tracted beam [72]: high stability, which allows greater control in terms of elec-tronic and less staff employed; high currents, which positively influence the treatment time, thanks to the consequent higher dose rate capability; continuous beam, which makes easier the exploitation of modern scanning techniques, such as the breathing active treatment and the repainting. The main disadvantages are the weight of the magnet system, which in case of heavy ion beams can reach hundred of tons, and the fixed extraction energy, which implies the use of pas-sive energy modulation. As already mentioned, paspas-sive systems cause the beam fragmentation and the consequently deterioration of the quality field. However, adequate technical solutions permit to partially solve this problem, especially from the radioprotection point of view, for example with the screening of the
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produced neutrons. On the other hand, synchrotrons are machine with a well-known technology, having low weight and power consumption. The main ad-vantage respect to cyclotrons is the possibility to directly modulate the energy of the extracted beam, which avoid the use of passive degradation. Anyway, they require a complex ion injector system, and their dimensions can reach 30 m in diameter, in case of acceleration of heavy ions. They produce pulsed beams with relatively low intensity.
As concern the cost, it is a widespread opinion that the synchrotron is a more expensive machine, but it must be taken into account some other elements which can heavily contribute to the total cost of an hadrontherapy facility (for example the gantry), especially in case of heavy ion centres [73].