Instrumentation & Measurement Magazine 24-9 - 66

provided by the motion detection interface, are added to the
planned position within the StrpFPGA feedback loop. This allows
for fast adjustments to the lateral beam position while
maintaining the position feedback functionalities to the scanning
system. Optimal parameters for the update frequency of
tracking vectors are still under investigation and will be validated
experimentally. Range tracking [30] may still be possible
but has not been implemented, due to the challenges with safe
clinical use.
The multi-phase 4D delivery strategy was also implemented
in the DDS for conformal motion compensation. This
method uses detected tumor motion to redirect the plan delivery
sequence, rather than modifying the beam spot delivery
positions. This is possible through the use of 4D plan libraries.
The plan libraries are created with 4D-CT images. The 4DCT
images are divided in different phases, where the tumor
is located in different positions in the breathing cycle. Then
a treatment plan is created on each motion phase in the 4DCT,
by separately optimizing one dedicated treatment plan on
each motion phase [27], [31]. The plan optimizations are independent
from each other, and the sum of all plans is a complete
treatment that corresponds to the prescription dose. The 4Doptimization
planning process incorporates changes to the
patient anatomy into the treatment plan, including the correct
beam ranges for each of the plans. As such, in the absence
of large deviations from motion trajectories found on the 4DCT
images, online range changes are not necessary to achieve
conformal dose coverage. The delivery of plan libraries requires
motion-synchronization, where the DDS continuously
switches between treatment plans according to the detected
motion received by memoFPGA. This continues until all plans
are delivered to completion.
Beam tracking offers flexibility during delivery and can
react to variable motion; however, it is challenged by complex
motion patterns and motion-induced range changes. In
contrast, 4D-optimization can incorporate a priori available information
on target motion, including range but relies on the
reproducibility of breathing motion during delivery. The tradeoff
of these characteristics as well as synergies of a combined
strategy will be explored in future experiments using the DDS.
A series of validation experiments has been carried out at
CNAO and GSI [19], [32]. An example is given in Fig. 4, showing
dose measurements performed at CNAO with an IC array
detector (Octavius XDR 1500; PTW, Freiburg, Germany) overlayed
on the planned dose distributions for a static treatment.
The target was a 60×60×60 mm3
cube, centered at 100 mm water-equivalent
depth and planned to a homogeneous dose
of 3 Gy. For the multi-phase 4D-delivery, the detector was
mounted onto a linear stage, which moved with a 22 mm sinusoidal
motion trajectory, perpendicular to the beam direction.
Water-equivalent plastic slabs (RW3, LAP GmbH, Lüneburg,
Germany) were used to position the detector in the center of
the cube. For reference measurements, a static treatment delivery
was also acquired by delivering a static treatment plan
(planned on one motion phase of the 4D-CT) to a stationary
detector. The same plan was also delivered to the moving detector
to estimate the impact of interplay ( " uncompensated " ).
For the multi-phase 4D deliveries, the motion was divided into
six phases, and the plan library was delivered using the described
motion-synchronization ( " six-phases " ) approach. A
3D-gamma analysis [33] (with pass criteria of 3 mm distance to
agreement and 3 % dose difference) showed pass rates of 32%,
92.3% and 100% for the uncompensated delivery, six-phases
delivery and static delivery, respectively. The test results are
shown in Fig. 4.
Integration of the RIDOS System for Online Dose
Reconstruction
The aim of this project is to provide online feedback to the
operators on the dose delivered in the presence of motion.
This permits both supervision of the delivery and an ad hoc
judgment on the effectiveness of chosen motion mitigation
strategies as well as a consideration of reconstructed 4Ddoses
in following treatment fractions in an adaptive therapy
scheme.
A rapid software tool for dose reconstruction was developed
in the framework of the RIDOS INFN project [20]. This
tool exploits the use of a Graphic Processing Unit (GPU) and
a Treatment Planning System, developed in-house, for proton
and carbon ion therapy [34]. The goal of RIDOS was to develop
Fig. 4. Measurements of a 60×60×60 mm3
66
cube with an IC array detector (white squares) over the planned static dose. For uncompensated and six-phase delivery,
the detector moved with a 22 mm amplitude sinusoidal motion during treatment delivery.
IEEE Instrumentation & Measurement Magazine
December 2021

Instrumentation & Measurement Magazine 24-9

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