11 Application of Accelerators and Storage Rings
735
upstream of the patient. The time needed to move the beam transversally, following
the indication of a precise feedback system, is of the order of milliseconds.
Due to the respiration cycle, the target organs can also move longitudinally,
i.e. in the direction of the beam. A good example of the problems concerning the
movements is a rib which enters the irradiation field of a lung tumour. In cyclotrons
and synchrotrons the time needed to change the energy, and thus the depth of the
Bragg peak, is of the order of 1 s because of the delay necessary either to move
energy absorbers (in cyclotrons) or to change the energy (in synchrotrons). An
accelerator with an energy cycle of the order of a few milliseconds would allow
a longitudinal follow-up of the tumour similar to the transversal one.
In synchrotrons, the beam is delivered to the treatment rooms only for fraction
of the machine time. Ramping up and down the magnets after the extraction of the
beam and waiting for the magnetic field to stabilize are time-consuming operations.
Few seconds are needed in this procedure and to reduce this dead time the centre
of HIT studied a system in which probes for the measurements of the field in the
magnet were inserted on the feed-back loop of the regulation of the magnetic fields
in the elements of synchrotron and beam lines. The result was an overall increase
in efficiency of 24% and a reduction of the time between two successive extractions
to approximately 1 s. If such time intervals are still not compatible with movement
compensation, nevertheless the improved efficiency has important impacts on the
facility, since a larger number of patients can be treated and the cost per treatment
decreases [95].
NIRS and HIMAC optimized the efficiency of their extraction system allowing
to extract of multiple energies, during a single machine cycle [96]. It is important to
remind that, in a treatment based on active scanning, the first layer irradiated is the
deepest, and then step by step all other layers. The number of particles accelerated
to certain energy in general exceeds the total number needed for irradiating the
corresponding layer at the planned dose. The beam not used is dumped and this
corresponds also to a waste of time. At NIRS, a system was studied to avoid
beam dumping using the synchrotron to decelerate and use the remaining beam
to the next more superficial layer. The process takes approximately 100 ms and,
since the procedure can be repeated several times within the same beam cycle, the
optimization of the irradiation time is consistent. Similar techniques are feasible to
compensate tumour movements.
Recently two new fast cycling accelerators—particularly suited for the movement compensation and repainting of moving organs—have been considered: high
frequency linacs and Fixed Field Alterenating Gradient (FFAG) accelerators.
Since 1993 the linac approach is pursued by the TERA Foundation with the
choice of using the same frequency (3 GHz) employed in the construction of
electron medical linacs [97]. Since the acceleration of very low velocity protons (and
carbon ions) is problematic, it was proposed to use as injector a commercial 30 MeV
cyclotron. The cyclotron-linac complex has been dubbed ‘cyclinac’. Between 1998
and 2003 a TERA, CERN, INFN collaboration built and tested a 3 GHz 1-m long
side-coupled standing-wave structure which accelerated protons from 62 to 74 MeV
[98] and could stand gradients as large as 27 MV/m, corresponding to surface
735
upstream of the patient. The time needed to move the beam transversally, following
the indication of a precise feedback system, is of the order of milliseconds.
Due to the respiration cycle, the target organs can also move longitudinally,
i.e. in the direction of the beam. A good example of the problems concerning the
movements is a rib which enters the irradiation field of a lung tumour. In cyclotrons
and synchrotrons the time needed to change the energy, and thus the depth of the
Bragg peak, is of the order of 1 s because of the delay necessary either to move
energy absorbers (in cyclotrons) or to change the energy (in synchrotrons). An
accelerator with an energy cycle of the order of a few milliseconds would allow
a longitudinal follow-up of the tumour similar to the transversal one.
In synchrotrons, the beam is delivered to the treatment rooms only for fraction
of the machine time. Ramping up and down the magnets after the extraction of the
beam and waiting for the magnetic field to stabilize are time-consuming operations.
Few seconds are needed in this procedure and to reduce this dead time the centre
of HIT studied a system in which probes for the measurements of the field in the
magnet were inserted on the feed-back loop of the regulation of the magnetic fields
in the elements of synchrotron and beam lines. The result was an overall increase
in efficiency of 24% and a reduction of the time between two successive extractions
to approximately 1 s. If such time intervals are still not compatible with movement
compensation, nevertheless the improved efficiency has important impacts on the
facility, since a larger number of patients can be treated and the cost per treatment
decreases [95].
NIRS and HIMAC optimized the efficiency of their extraction system allowing
to extract of multiple energies, during a single machine cycle [96]. It is important to
remind that, in a treatment based on active scanning, the first layer irradiated is the
deepest, and then step by step all other layers. The number of particles accelerated
to certain energy in general exceeds the total number needed for irradiating the
corresponding layer at the planned dose. The beam not used is dumped and this
corresponds also to a waste of time. At NIRS, a system was studied to avoid
beam dumping using the synchrotron to decelerate and use the remaining beam
to the next more superficial layer. The process takes approximately 100 ms and,
since the procedure can be repeated several times within the same beam cycle, the
optimization of the irradiation time is consistent. Similar techniques are feasible to
compensate tumour movements.
Recently two new fast cycling accelerators—particularly suited for the movement compensation and repainting of moving organs—have been considered: high
frequency linacs and Fixed Field Alterenating Gradient (FFAG) accelerators.
Since 1993 the linac approach is pursued by the TERA Foundation with the
choice of using the same frequency (3 GHz) employed in the construction of
electron medical linacs [97]. Since the acceleration of very low velocity protons (and
carbon ions) is problematic, it was proposed to use as injector a commercial 30 MeV
cyclotron. The cyclotron-linac complex has been dubbed ‘cyclinac’. Between 1998
and 2003 a TERA, CERN, INFN collaboration built and tested a 3 GHz 1-m long
side-coupled standing-wave structure which accelerated protons from 62 to 74 MeV
[98] and could stand gradients as large as 27 MV/m, corresponding to surface