177 research outputs found
Synthesis of Covert Actuator Attackers for Free
In this paper, we shall formulate and address a problem of covert actuator
attacker synthesis for cyber-physical systems that are modelled by
discrete-event systems. We assume the actuator attacker partially observes the
execution of the closed-loop system and is able to modify each control command
issued by the supervisor on a specified attackable subset of controllable
events. We provide straightforward but in general exponential-time reductions,
due to the use of subset construction procedure, from the covert actuator
attacker synthesis problems to the Ramadge-Wonham supervisor synthesis
problems. It then follows that it is possible to use the many techniques and
tools already developed for solving the supervisor synthesis problem to solve
the covert actuator attacker synthesis problem for free. In particular, we show
that, if the attacker cannot attack unobservable events to the supervisor, then
the reductions can be carried out in polynomial time. We also provide a brief
discussion on some other conditions under which the exponential blowup in state
size can be avoided. Finally, we show how the reduction based synthesis
procedure can be extended for the synthesis of successful covert actuator
attackers that also eavesdrop the control commands issued by the supervisor.Comment: The paper has been accepted for the journal Discrete Event Dynamic
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Validation and clinical implementation of an accurate Monte Carlo code for pencil beam scanning proton therapy.
Monte Carlo (MC)-based dose calculations are generally superior to analytical dose calculations (ADC) in modeling the dose distribution for proton pencil beam scanning (PBS) treatments. The purpose of this paper is to present a methodology for commissioning and validating an accurate MC code for PBS utilizing a parameterized source model, including an implementation of a range shifter, that can independently check the ADC in commercial treatment planning system (TPS) and fast Monte Carlo dose calculation in opensource platform (MCsquare). The source model parameters (including beam size, angular divergence and energy spread) and protons per MU were extracted and tuned at the nozzle exit by comparing Tool for Particle Simulation (TOPAS) simulations with a series of commissioning measurements using scintillation screen/CCD camera detector and ionization chambers. The range shifter was simulated as an independent object with geometric and material information. The MC calculation platform was validated through comprehensive measurements of single spots, field size factors (FSF) and three-dimensional dose distributions of spread-out Bragg peaks (SOBPs), both without and with the range shifter. Differences in field size factors and absolute output at various depths of SOBPs between measurement and simulation were within 2.2%, with and without a range shifter, indicating an accurate source model. TOPAS was also validated against anthropomorphic lung phantom measurements. Comparison of dose distributions and DVHs for representative liver and lung cases between independent MC and analytical dose calculations from a commercial TPS further highlights the limitations of the ADC in situations of highly heterogeneous geometries. The fast MC platform has been implemented within our clinical practice to provide additional independent dose validation/QA of the commercial ADC for patient plans. Using the independent MC, we can more efficiently commission ADC by reducing the amount of measured data required for low dose "halo" modeling, especially when a range shifter is employed
Networked Supervisor Synthesis Against Lossy Channels with Bounded Network Delays as Non-Networked Synthesis
In this work, we study the problem of supervisory control of networked
discrete event systems. We consider lossy communication channels with bounded
network delays, for both the control channel and the observation channel. By a
model transformation, we transform the networked supervisor synthesis problem
into the classical (non-networked) supervisor synthesis problem (for
non-deterministic plants), such that the existing supervisor synthesis tools
can be used for synthesizing networked supervisors. In particular, we can use
the (state-based) normality property for the synthesis of the supremal
networked supervisors, whose existence is guaranteed by construction due to our
consideration of command non-deterministic supervisors. The effectiveness of
our approach is illustrated on a mini-guideway example that is adapted from the
literature, for which the supremal networked supervisor has been synthesized in
the synthesis tools SuSyNA and TCT.Comment: This paper is under review for Automatic
Characterization of 250 MeV protons from Varian ProBeam pencil beam scanning system for FLASH radiation therapy
Recently, shoot-through proton FLASH has been proposed where the highest
energy is extracted from the cyclotron to maximize the dose rate (DR). Even
though our proton pencil beam scanning system can deliver 250 MeV (the highest
energy), it is not typical to use 250 MeV protons for routine clinical
treatments and as such 250 MeV may not have been characterized in the
commissioning. In this study, we aim to characterize 250 MeV protons from
Varian ProBeam system for FLASH RT as well as assess the ability of clinical
monitoring ionization chamber (MIC) for FLASH-readiness. We measured data
needed for beam commissioning: integral depth dose (IDD) curve, spot sigma, and
absolute dose calibration. To evaluate MIC, we measured output as a function of
beam current. To characterize a 250 MeV FLASH beam, we measured: (1) central
axis DR as a function of current and spot spacing and arrangement, (2) for a
fixed spot spacing, the maximum field size that still achieves FLASH DR (i.e.,
> 40 Gy/s), (3) DR reproducibility. All FLASH DR measurements were performed
using ion chamber for the absolute dose and irradiation times were obtained
from log files. We verified dose measurements using EBT-XD films and
irradiation times using a fast, pixelated spectral detector. R90 and R80 from
IDD were 37.58 and 37.69 cm, and spot sigma at isocenter were {\sigma}x=3.336
and {\sigma}y=3.332 mm, respectively. The absolute dose output was measured as
0.377 GyE*mm2/MU for the commissioning conditions. Output was stable for beam
currents up to 15 nA, and it gradually increased to 12-fold for 115 nA. DR
depended on beam current, spot spacing and arrangement and could be reproduced
within 4.2% variations. Even though FLASH was achieved and the largest field
size that delivers FLASH DR was determined as 35x35 mm2, current MIC has DR
dependence and users should measure DR each time for their FLASH applications.Comment: 11 pages, 6 figure
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