Biomedical Engineering Reference
In-Depth Information
generally a fixed energy machine and, to produce protons of less
energy, the energy of the protons must be adjusted downstream of the
accelerator. This is done with a variable-thickness degrader, which
interposes varying amounts of material in the beam, thus altering its
residual penetration. The process of reducing the energy in this
manner also scatters the beam (via multiple Coulomb scattering in the
degrader) by a significant amount and leaves the beam with a spread
of energies due to range straggling. This spreading in both angle and
energy is repaired using collimators and bending magnets which act
as a spectrometer, picking out a narrow band of energies with
adequately small size and divergence of the resultant proton beam.
The process of energy degradation is quite inefficient; in the extreme
as much as 99% of the protons may be stopped in collimators and
thus be “lost” from the useful beam. As a result: (a) the cyclotron
must be capable of producing substantially more intense beams than a
synchrotron; and (b) additional shielding is required to shield against
the neutrons produced by the lost protons. Generally, these neutrons
are produced far from the patient and, with appropriate shielding, do
not contribute significantly to the neutron dose that he or she receives.
The time structure of the beam from the two accelerators is also
different. A sector-focused cyclotron produces a virtually continuous
beam, whereas the synchrotron delivers its protons in pulses, usually
of a few seconds duration and with a few seconds dead time in-
between the pulses. The pulse structure of synchrotrons is a com-
plicating factor for the implementation of repainting and beam gating
(see below).
An active debate goes on between proponents of the two types of
accelerator. I will not jump into this debate; both can do the job.
Beam-transport system
Protons must be brought from the accelerator to the treatment delivery
device. This is done using magnets to guide the beam using the same
principle as allows an electric motor to work, namely the fact
(discovered by Faraday in 1821) that a moving charged particle
experiences a lateral force when moving through a magnetic field.
Protons, since they have to be transported long distances and need
magnetic lenses to keep the beam size small enough, use many
magnets, linacs generally only one.
