Instrumentation & Measurement Magazine 24-9 - 65

control loops. In addition to the spot-by-spot control, the PXI
controller establishes a large number of interfaces to devices,
systems, and services in the accelerator complex to manage
treatment delivery. These interfaces are shown in Fig. 3.
Other components mounted in the cabinet include the insulation
transformer, the low voltage power supply (to power
the monitors), the gas control unit, the monitor data transmission
module, the interface toward interlock system, and a
module that records the treatment status in case of power failure.
From this module, the plan delivery can be completed
seamlessly once full operation is restored.
DDS as a Source of Information
The DDS can be used as a source of treatment delivery information,
because it controls and monitors treatments and
records the beam intensity and positions in real-time. The online
DDS data is necessary for research projects involving new
detectors, dosimeters and online tools. For this reason, optical
interfaces have been created to make several treatment
progression signals available to external devices. These signals
include a trigger signal that is generated at the end of each
spot and a serial stream that is generated to send slice and spot
numbers at the beginning of a spot. Moreover, a continuous
serial stream of data is generated to send the digital data produced
by intensity monitor to implement a feedback loop that
controls the beam intensity.
An external system can use this data and the planned spot
sequence to add spatial and intensity information in its online
data and logfiles. As an example, this information is crucial for
the PET-based range monitor INSIDE that is used at CNAO
during treatment for selected patients. The in-beam PET
scanner needs the beam spatial and intensity information in
real-time for the on-the-fly reconstruction of the activity distribution
of secondary particles. An automated time-resolved
quantitative analysis reconstructs the 3D activity map every
10 s to provide the actual beam range in the patient [17]. The
PET signal provides important feedback on the range of the
beam in the patient. This parameter is of highest importance
for the actual dose distribution achieved by particle beams;
however, it is associated with various uncertainties ranging
from the conversion of CT images into beam range to anatomical
changes under therapy. PET imaging is an important tool
for quality assurance and increased reliability of the dose delivered
over the course of a treatment.
Future 4D Technologies for the CNAO
DDS
Moving tumors remain the leading challenge for particle therapy.
Advanced stage disease of lung, liver and pancreas offer
a particularly poor prognosis without good treatment options,
and protons or heavier ions could provide a means for
improved therapy, without dose limiting toxicity as is the
case for conventional radiotherapy [23]. However, reliably
mitigating for tumor motion is still a challenge for scanned
ion therapy [24], and anatomical changes during the treatment
delivery could take advantage from online methods and
December 2021
4D-technologies to guarantee target coverage. Therefore, the
DDS was expanded to provide modular options for tumor motion
detection and mitigation [18].
Assuming regular motion, the time resolved computed tomography
(4D-CT) is still the best surrogate for online patient
imaging and therefore the standard for 4D-therapy. Different
4D solutions were investigated at CNAO, which include specific
hardware and software improvements and upgrades to
the DDS. Two examples are described in the next sections.
Realtime Plan Adaptation
The aim of this project is to control and mitigate the effect of
motion on the scanned dose delivery: restoring sharp dose
gradients, eliminating the interplay effect, and delivering the
beam conformally to the target in spite of motion-induced
range changes [25].
This extension currently is only available as a research
version and is not licensed for clinical use. It is used in the experimental
room in CNAO and at the German Ion Research
Center (GSI) in Darmstadt, Germany. It should be noted that
GSI is a research facility that introduced carbon ion therapy
in Europe but is no longer involved in patient treatment. The
DDS was installed in the former therapy cave, where it is used
both for therapy research as described here, but also in various
radiobiological experiments. For the latter, the DDS successfully
delivered a variety of primary and secondary scanned
ion beams, including protons, carbon, and oxygen, but also
15
O, as well as heavier ions for space applications, such as calcium
and iron. This version is a copy of the DDS used clinically
at CNAO, where all NI hardware has been updated to newer
and higher performing solutions. New FPGA cards have more
gates and on-board memory, and the crate and controller are
now upgraded to PXI Express technologies. Moreover, a new
FPGA card (MemoFPGA) has been added to replace the four
memory cards. The plan is loaded inside the memory of the
memoFPGA during treatment preparation phase, before treatment
begins, to adapt the beam spot delivery sequence on the
fly through external signals.
These signals are produced by a device that is the interface
between a motion detection device and the DDS. This
allows for online motion monitoring and provides log-files
with motion information for 4D-dose reconstructions [26]. The
interface can accept digital and analog inputs from motion
detection devices [27] in the form of motion states, position
vectors or a (multi-dimensional) motion trace. The motion
trace can be converted into an amplitude or phase-based
motion phase as needed for the selected motion mitigation
strategy. Several motion mitigation strategies have been
implemented, such as gating (pausing irradiation when the tumor
is outside the planned treatment position), beam tracking
and multi-phase 4D delivery of treatment plan libraries [25].
The beam tracking method was implemented following a
similar strategy as in the previous GSI treatment control system
[28], [29]. With the tracking method, the tumor position
is sent to the DDS (MemoFPGA), and the beam is redirected
to compensate for the tumor motion. Lateral tracking vectors,
IEEE Instrumentation & Measurement Magazine
65

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