A study of light ion accelerators for cancer treatment Page: 18 of 34
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increments. This approach is justified because of the need to establish proper protocols for the
treatment and because of high values of the factor RBE for some tissues such as the central nervous
system which have to be spared as much as possible. Preliminary results show that there were no
major healthy tissue morbidities and that carbon ion therapy is a promising modality for cancer
treatment. Trials are planned to continue, expanding eventually the range of ion species to silicon
or argon (for tumors located closer to the surface), with the hope to determine the appropriate role
of ions in radiotherapy.
At the GSI heavy ion research complex in Darmstadt (Germany) an experimental cancer
treatment program is underway, to continue for five years and to include about 350 patients [44][45].
The main objective of the program is to test a novel, most advanced method of beam delivery by
using two-dimensional magnetic raster scanning coupled with an active accelerator energy
modulation. An ionization chamber in front of the patient measures the number of ions at a specific
point in the tumor volume and controls the speed of scanning. After the successful conclusion of the
clinical trials the plan is to design and build a hospital based facility.
TYPES OF ACCELERATORS FOR LIGHT ION THERAPY
Cyclotrons
Cyclotrons are machines with a constant magnetic field and a fixed frequency of the
accelerating voltage. The injection of the beam from the ion source, its acceleration in the machine
and ejection are a continuous process; the extracted beam has a fixed energy and its intensity can
also be continuous which can have advantages when scanning the tumor. While cyclotrons delivering
proton beams with energies up to 230 MeV have already been developed by industry to operate in
a hospital environment, their application as accelerators for light ion therapy is not very feasible. The
energy per nucleon needed for the same penetration depth is higher, also the charge to mass ratio of
ions compared to protons is lower; because of these factors a standard design light ion cyclotron for
cancer treatment would have a prohibitively large magnet. The only cyclotron even considered for
light ions was part of the now abandoned EULIMA (EUropean Light Ion Medical Accelerator)
project. In order to reduce the large size and weight of a standard magnet, a superconducting single
coil design was considered and developed for EULIMA, having an external radius of only 2.32 m.
However, the cyclotron was not the preferred choice for this facility because the required
superconducting technology was very sophisticated, not justifying other advantages of a cyclotron.
Linear Accelerators
Conventional linear accelerators are usually very low duty factor machines, delivering high
ion beam currents in short pulses (millisecond or so duration), often for injection into the next stage
accelerator such as a synchrotron. They can accept and accelerate ions having a certain ratio of the
charge to mass and deliver a beam with an energy fixed or, at best, variable in large steps. Although
the extraction efficiency is close to 100%, there are presently no linear accelerators used for either
proton or light ion therapy (there was a proposal to use a small fraction of the proton beam from the
Brookhaven National Laboratory's 200 MeV linear accelerator for cancer treatment but it was
decided not to proceed). Linear accelerators are machines requiring a large space, they are expensive
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Prelec, K. A study of light ion accelerators for cancer treatment, report, July 1, 1997; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc691514/m1/18/: accessed May 10, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.