Instrumentation & Measurement Magazine 24-9 - 63

optimization of multiple fields enables so-called intensity
modulated particle therapy (IMPT) to selectively spare critical
risk structures.
The increased precision of particle beams also comes at
the cost of higher demands to treatment accuracy. This is especially
true when using highly conformal scanned beams,
which are delivered sequentially instead of irradiating the entire
tumor continuously. Significant research efforts have been
made and are still ongoing to reach the theoretically possible
precision of particle beams.
To assure that pencil beams are delivered with clinical accuracy
and meet proper safety standards, treatment beam
lines are equipped with a dose delivery system (DDS) that
contains the instrumentation for guiding and controlling particle
beams during treatment. Such a DDS for PBS mainly
comprises detectors for real-time beam monitoring [5] and a
pair of orthogonal scanning magnets to move (or deflect) the
beam in vertical and horizontal directions [6], [7]. To adjust the
Bragg peak position in depth, synchrotron-based therapy systems
can vary the beam energy from spill-by-spill. In contrast,
cyclotrons provide fixed beam energies that are modulated
passively with range shifters in the beam line. The DDS directs
treatment delivery according to the sequence of pencil
beams defined by a treatment planning system (TPS). Each
pencil beam is characterized by the number of particles, the
lateral position (x and y) and the energy [8]. The beam fluence
and position are measured in real-time with dedicated beam
monitors, while the beam is deflected to the planned positions.
The scanning magnets are signalled to deflect the beam
as soon as the prescribed number of particles for the current
beam position has been reached. The next beam energy is set
when the DDS signals that all particles at the given energy
have been delivered.
Currently, range uncertainties when delivering scanned
beams, caused by issues such as organ motion and anatomy
variations, affect the quality of the treatment deliveries [9],
[10]. Mitigation strategies were developed [11], [12], and several
research projects are ongoing to optimize the treatment
of disease sites that are more sensitive to anatomy variations
during treatment, including abdominal and thoracic tumors.
Towards this aim, new DDS features have been developed
within the existing delivery systems and others are currently
in development. These strategies aim to exploit the full benefit
of protons and heavier ions [13].
The main topic of this review is to describe the DDS of
the Italian National Center of Oncological Hadrontherapy
(CNAO) [14]-[16]. In the first section, the technologies used
to treat more than 3200 patients since 2011 are reviewed. Additionally,
details are provided on the treatment data from the
DDS to other systems, such as the range verification system
delivered in the INnovative Solution for In-beam Dosimetry
in hadronthErapy (INSIDE) project [17]. The second section
describes two 4D solutions investigated at CNAO to provide
the DDS with new features for treating moving targets:
multi-phase 4D dose delivery [18], [19] and online dose reconstruction
[20].
December 2021
Current Technologies of the CNAO DDS
The DDS is the unit of the accelerator complex that controls
and monitors treatment delivery. The CNAO DDS is a CE
marked medical device and is used for clinical treatments
in CNAO and in MedAustron EBG, since 2011 and 2016, respectively.
Both centers are based on the 'Proton-Ion Medical
Machine Study' (PIMMS) design, conducted at CERN. The
DDS was developed at CNAO to realize dose delivery to clinical
standards for the PIMMS machines. It mainly comprises
two beam monitors, mounted on the beam line at the end of
the vacuum pipe and a cabinet where the control electronics
are housed, located in a separate room.
Beam Monitors
The beam monitors are sealed transmission ionization chambers
with a large sensitive area to measure the pencil beam
throughout the course of treatment delivery. To meet the required
safety standards for redundancy, the two monitors are
independent units in the data acquisition system and are electrically
powered separately. The two monitoring units are
often referred to as BOX1 and BOX2.
Inside a BOX, multiple ionization chambers are mounted
to measure the number of particles delivered and the beam position.
The number of particles is determined by converting
the measured charge, released by the beam inside the sensitive
volume, and is collected by a large area electrode (integral
chamber). The beam position is measured in the horizontal
and vertical directions by a pair of segmented strip chambers
or by a pixel segmented ionization chamber.
The strip anode comprises 128 strips displaced by 1.65 mm,
and the pixel anode comprises 1024 pads arranged as a matrix
32×32 with a pitch of 6.6 mm. The insulation between strip
and pixel is 0.1 mm wide. The thickness of the electrodes (cathodes,
anodes) and gas volume containing plates inside the two
monitors is limited to a total water equivalent thickness of 0.9
mm to minimize the material that the beam traverses. The anode
and cathode comprise a thin aluminium layer deposited
on Mylar foils. To obtain strips and pixels, the anodes are produced
using printed circuit board production procedures.
To maintain the right gain and collection efficiency, the
two monitors are continuously flushed with Nitrogen. Gas
temperature and pressure are measured before each treatment
to calculate the environment calibration factor to convert
from collected charge to the number of particles. The charge
collected by the segmented anode is acquired directly by frontend
electronics, located inside the monitors by two VLSI chips
developed intentionally for this application [21], [22]. The
output signal is digital, and all of the electronics channels belonging
to a detector are read by the data acquisition programs
as memory blocks, containing an address that corresponds
to the position and the collected data that corresponds to the
charge collected by the single channel. The conversion to digital
signals inside the BOX gives very low noise levels and
simplifies the data acquisition. Digital data is transmitted in
parallel to the acquisition systems through optical decouplers
and high-speed differential lines.
IEEE Instrumentation & Measurement Magazine
63
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