Browsing by Subject "Monte Carlo"

Now showing 1 - 3 of 3
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    Challenges of dosimetry of ultra-short pulsed very high energy electron beams
    (Elsevier, 2017-10-01) Subiel, Anna; Moskvin, Vadim; Welsh, Gregor H.; Cipiccia, Silvia; Reboredo, David; DesRosiers, Colleen; Jaroszynski, Dino A.; Radiation Oncology, School of Medicine
    Very high energy electrons (VHEE) in the range from 100 to 250MeV have the potential of becoming an alternative modality in radiotherapy because of their improved dosimetric properties compared with 6–20MV photons generated by clinical linear accelerators (LINACs). VHEE beams have characteristics unlike any other beams currently used for radiotherapy: femtosecond to picosecond duration electron bunches, which leads to very high dose per pulse, and energies that exceed that currently used in clinical applications. Dosimetry with conventional online detectors, such as ionization chambers or diodes, is a challenge due to non-negligible ion recombination effects taking place in the sensitive volumes of these detectors. FLUKA and Geant4 Monte Carlo (MC) codes have been employed to study the temporal and spectral evolution of ultrashort VHEE beams in a water phantom. These results are complemented by ion recombination measurements employing an IBA CC04 ionization chamber for a 165MeV VHEE beam. For comparison, ion recombination has also been measured using the same chamber with a conventional 20MeV electron beam. This work demonstrates that the IBA CC04 ionization chamber exhibits significant ion recombination and is therefore not suitable for dosimetry of ultrashort pulsed VHEE beams applying conventional correction factors. Further study is required to investigate the applicability of ion chambers in VHEE dosimetry.
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    A perspective on the theoretical and numerical aspects of Ion Mobility Spectrometry
    (Elsevier, 2021-12) Larriba-Andaluz, Carlos; Mechanical Engineering, School of Engineering and Technology
    Ion Mobility Spectrometry (IMS) has become a ubiquitous analytical technique, in particular when used as an orthogonal technique to Mass Spectrometry (MS). As separations of ions in the gas phase become more precise, the need to provide a suitable theory that explains the observed differences is apparent. While the theory exists, much of it is obscured due to the difficulty of the equations and the approximations to the solution. This work explores some of the more useful theoretical approaches to IMS while making use of a full Monte Carlo simulations algorithm to provide some pedagogical examples that characterize the reasons behind the different theoretical approaches, and whether they need to be used for a particular calculation. To improve the existing theory, reliable empirical data is required. For such reason, an appropriate labeling system for mobility is proposed here requiring that at least the temperature, gas, electric field, and instrument employed are provided and which is an extension of the previous protocol.
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    Variation of kQclin,Qmsr (fclin,fmsr) for the small-field dosimetric parameters percentage depth dose, tissue-maximum ratio, and off-axis ratio
    (Wiley, 2014-10) Francescon, Paolo; Beddar, Sam; Satariano, Ninfa; Das, Indra J.; Department of Radiation Oncology, IU School of Medicine
    Purpose: Evaluate the ability of different dosimeters to correctly measure the dosimetric parameters percentage depth dose (PDD), tissue-maximum ratio (TMR), and off-axis ratio (OAR) in water for small fields. Methods: Monte Carlo (MC) simulations were used to estimate the variation of kfclin,fmsrQclin,Qmsr for several types of microdetectors as a function of depth and distance from the central axis for PDD, TMR, and OAR measurements. The variation of kfclin,fmsrQclin,Qmsr enables one to evaluate the ability of a detector to reproduce the PDD, TMR, and OAR in water and consequently determine whether it is necessary to apply correction factors. The correctness of the simulations was verified by assessing the ratios between the PDDs and OARs of 5- and 25-mm circular collimators used with a linear accelerator measured with two different types of dosimeters (the PTW 60012 diode and PTW PinPoint 31014 microchamber) and the PDDs and the OARs measured with the Exradin W1 plastic scintillator detector (PSD) and comparing those ratios with the corresponding ratios predicted by the MC simulations. Results: MC simulations reproduced results with acceptable accuracy compared to the experimental results; therefore, MC simulations can be used to successfully predict the behavior of different dosimeters in small fields. The Exradin W1 PSD was the only dosimeter that reproduced the PDDs, TMRs, and OARs in water with high accuracy. With the exception of the EDGE diode, the stereotactic diodes reproduced the PDDs and the TMRs in water with a systematic error of less than 2% at depths of up to 25 cm; however, they produced OAR values that were significantly different from those in water, especially in the tail region (lower than 20% in some cases). The microchambers could be used for PDD measurements for fields greater than those produced using a 10-mm collimator. However, with the detector stem parallel to the beam axis, the microchambers could be used for TMR measurements for all field sizes. The microchambers could not be used for OAR measurements for small fields. Conclusions: Compared with MC simulation, the Exradin W1 PSD can reproduce the PDDs, TMRs, and OARs in water with a high degree of accuracy; thus, the correction used for converting dose is very close to unity. The stereotactic diode is a viable alternative because it shows an acceptable systematic error in the measurement of PDDs and TMRs and a significant underestimation in only the tail region of the OAR measurements, where the dose is low and differences in dose may not be therapeutically meaningful.