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Sökning: L773:0094 2405 OR L773:2473 4209 > (2010-2014)

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2.
  • Benmakhlouf, Hamza, et al. (författare)
  • Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams : A PENELOPE Monte Carlo study
  • 2014
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 41:4, s. 041711-
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: To determine detector-specific output correction factors, k(Qclin,Qmsr)(fclin,fmsr) in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers. Methods: Field output factors, defined according to the international formalism published by Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 x 0.5 cm(2) and 10 x 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 x 10 cm(2). Results: The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about +/- 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or un-shielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates. Conclusions: The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model. (C) 2014 American Association of Physicists in Medicine.
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3.
  • Dasu, Alexandru, et al. (författare)
  • Impact of variable RBE on proton fractionation
  • 2013
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 40:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: To explore the impact of variable proton RBE on dose fractionation for clinically-relevant situations. A generic RBE=1.1 is generally used for isoeffect calculations, while experimental studies showed that proton RBE varies with tissue type, dose and LET.Material and methods: An analytical expression for the LET and α/β dependence of the LQ model has been used for proton simulations in parallel with the assumption of a generic RBE=1.1. Calculations have been performed for ranges of LET values and fractionation sensitivities to describe clinically-relevant cases, like the treatment of H&N and prostate tumors. Isoeffect calculations were compared with predictions from a generic RBE value and reported clinical results.Results: The generic RBE=1.1 appears to be a reasonable estimate for the proton RBE of rapidly growing tissues irradiated with low LET radiation. However, the use of a variable RBE predicts larger differences for tissues with low α/β (both tumor and normal) and at low doses per fraction. In some situations these differences may appear in contrast to the findings from photon studies highlighting the importance of accurate accounting for the radiobiological effectiveness of protons. Furthermore, the use of variable RBE leads to closer predictions to clinical results. Conclusions: The LET dependence of the RBE has a strong impact on the predicted effectiveness of fractionated proton radiotherapy. The magnitude of the effect is modulated by the fractionation sensitivity and the fractional dose indicating the need for accurate analyses both in the target and around it. Care should therefore be employed for changing clinical fractionation patterns or when analyzing results from clinical studies for this type of radiation.
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4.
  • Giantsoudi, D., et al. (författare)
  • A gEUD-based inverse planning technique for HDR prostate brachytherapy : Feasibility study
  • 2013
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 40:4, s. 041704-
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: The purpose of this work was to study the feasibility of a new inverse planning technique based on the generalized equivalent uniform dose for image-guided high dose rate (HDR) prostate cancer brachytherapy in comparison to conventional dose-volume based optimization. Methods: The quality of 12 clinical HDR brachytherapy implants for prostate utilizing HIPO (Hybrid Inverse Planning Optimization) is compared with alternative plans, which were produced through inverse planning using the generalized equivalent uniform dose (gEUD). All the common dose-volume indices for the prostate and the organs at risk were considered together with radiobiological measures. The clinical effectiveness of the different dose distributions was investigated by comparing dose volume histogram and gEUD evaluators. Results: Our results demonstrate the feasibility of gEUD-based inverse planning in HDR brachytherapy implants for prostate. A statistically significant decrease in D-10 or/and final gEUD values for the organs at risk (urethra, bladder, and rectum) was found while improving dose homogeneity or dose conformity of the target volume. Conclusions: Following the promising results of gEUD-based optimization in intensity modulated radiation therapy treatment optimization, as reported in the literature, the implementation of a similar model in HDR brachytherapy treatment plan optimization is suggested by this study. The potential of improved sparing of organs at risk was shown for various gEUD-based optimization parameter protocols, which indicates the ability of this method to adapt to the user's preferences.
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8.
  • Lagerlöf, Jakob Heydorn, 1978, et al. (författare)
  • The impact of including spatially longitudinal heterogeneities of vessel oxygen content and vascular fraction in 3D tumor oxygenation models on predicted radiation sensitivity.
  • 2014
  • Ingår i: Medical physics. - : Wiley. - 0094-2405 .- 2473-4209. ; 41:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Oxygen distribution models have been used to analyze the influences of oxygen tensions on tissue response after radiotherapy. These distributions are often generated assuming constant oxygen tension in the blood vessels. However, as red blood cells progress through the vessels, oxygen is continuously released into the plasma and the surrounding tissue, resulting in longitudinally varying oxygen levels in the blood vessels. In the present study, the authors investigated whether a tumor oxygenation model that incorporated longitudinally varying oxygen levels would provide different predictions of necrotic fractions and radiosensitivity compared to commonly used models with a constant oxygen pressure.
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9.
  • Larsson, Daniel H., et al. (författare)
  • First application of liquid-metal-jet sources for small-animal imaging : High-resolution CT and phase-contrast tumor demarcation
  • 2013
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 40:2, s. 021909-
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: Small-animal studies require images with high spatial resolution and high contrast due to the small scale of the structures. X-ray imaging systems for small animals are often limited by the microfocus source. Here, the authors investigate the applicability of liquid-metal-jet x-ray sources for such high-resolution small-animal imaging, both in tomography based on absorption and in soft-tissue tumor imaging based on in-line phase contrast. Methods: The experimental arrangement consists of a liquid-metal-jet x-ray source, the small-animal object on a rotating stage, and an imaging detector. The source-to-object and object-to-detector distances are adjusted for the preferred contrast mechanism. Two different liquid-metal-jet sources are used, one circulating a Ga/In/Sn alloy and the other an In/Ga alloy for higher penetration through thick tissue. Both sources are operated at 40-50 W electron-beam power with similar to 7 mu m x-ray spots, providing high spatial resolution in absorption imaging and high spatial coherence for the phase-contrast imaging. Results: High-resolution absorption imaging is demonstrated on mice with CT, showing 50 mu m bone details in the reconstructed slices. High-resolution phase-contrast soft-tissue imaging shows clear demarcation of mm-sized tumors at much lower dose than is required in absorption. Conclusions: This is the first application of liquid-metal-jet x-ray sources for whole-body small-animal x-ray imaging. In absorption, the method allows high-resolution tomographic skeletal imaging with potential for significantly shorter exposure times due to the power scalability of liquid-metal-jet sources. In phase contrast, the authors use a simple in-line arrangement to show distinct tumor demarcation of few-mm-sized tumors. This is, to their knowledge, the first small-animal tumor visualization with a laboratory phase-contrast system.
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10.
  • Liamsuwan, Thiansin, et al. (författare)
  • Microdosimetry of proton and carbon ions
  • 2014
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 41:8, s. 239-250
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: To investigate microdosimetry properties of 160 MeV/u protons and 290 MeV/u C-12 ion beams in small volumes of diameters 10-100 nm. Methods: Energy distributions of primary particles and nuclear fragments in the beams were calculated from simulations with the general purpose code SHIELD-HIT, while energy depositions by monoenergetic ions in nanometer volumes were obtained from the event-by-event Monte Carlo track structure ion code PITS99 coupled with the electron track structure code KURBUC. Results: The results are presented for frequencies of energy depositions in cylindrical targets of diameters 10-100 nm, dose distributions (y) over bar (D) in lineal energy y, and dose-mean lineal energies YD For monoenergetic ions, the hp was found to increase with an increasing target size for high-linear energy transfer (LET) ions, but decrease with an increasing target size for low-LET ions. Compared to the depth dose profile of the ion beams, the maximum of the hp depth profile for the 160 MeV proton beam was located at similar to 0.5 cm behind the Bragg peak maximum, while the PD peak of the 290 MeV/u C-12 beam coincided well with the peak of the absorbed dose profile. Differences between the (y) over bar (D) and dose-averaged linear energy transfer (LETD) were large in the proton beam for both target volumes studied, and in the C-12 beam for the 10 nm diameter cylindrical volumes. The (y) over bar (D) determined for 100 run diameter cylindrical volumes in the C-12 beam was approximately equal to the LETD. The contributions from secondary particles to the (y) over bar (D) of the beams are presented, including the contributions from secondary protons in the proton beam and from fragments with atomic number Z = 1-6 in the C-12 beam. Conclusions: The present investigation provides an insight into differences in energy depositions in subcellular-size volumes when irradiated by proton and carbon ion beams. The results are useful for characterizing ion beams of practical importance for biophysical modeling of radiation-induced DNA damage response and repair in the depth profiles of protons and carbon ions used in radiotherapy.
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