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      Effect of secondary electron generation on dose enhancement in Lipiodol with and without a flattening filter

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          Abstract

          Purpose

          Lipiodol, which was used in transcatheter arterial chemoembolization before liver stereotactic body radiation therapy ( SBRT), remains in SBRT. Previous we reported the dose enhancement in Lipiodol using 10 MV (10×) FFF beam. In this study, we compared the dose enhancement in Lipiodol and evaluated the probability of electron generation ( PEG) for the dose enhancement using flattening filter ( FF) and flattening filter free ( FFF) beams.

          Methods

          FF and FFF for 6 MV (6×) and 10× beams were delivered by TrueBeam. The dose enhancement factor ( DEF), energy spectrum, and PEG was calculated using Monte Carlo ( MC) code BEAMnrc and heavy ion transport code system ( PHITS).

          Results

          DEFs for FF and FFF 6× beams were 7.0% and 17.0% at the center of Lipiodol (depth, 6.5 cm). DEFs for FF and FFF 10× beams were 8.2% and 10.5% at the center of Lipiodol. Spectral analysis revealed that the FFF beams contained more low‐energy (0–0.3 MeV) electrons than the FF beams, and the FF beams contained more high‐energy (>0.3 MeV) electrons than the FFF beams in Lipiodol. The difference between FFF and FF beam DEFs was larger for 6× than for 10×. This occurred because the 10× beams contained more high‐energy electrons. The PEGs for photoelectric absorption and Compton scattering for the FFF beams were higher than those for the FF beams. The PEG for the photoelectric absorption was higher than that for Compton scattering.

          Conclusions

          FFF beam contained more low‐energy photons and it contributed to the dose enhancement. Energy spectra and PEGs are useful for analyzing the mechanisms of dose enhancement.

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          Most cited references 21

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          BEAM: a Monte Carlo code to simulate radiotherapy treatment units.

          This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high-energy electron and photon beams, 60Co beams and orthovoltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75-20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed.
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            Current status and future perspective of flattening filter free photon beams.

            Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook. A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection. The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance. New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.
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              Stereotactic body radiation therapy in multiple organ sites.

              Stereotactic body radiation therapy (SBRT) uses advanced technology to deliver a potent ablative dose to deep-seated tumors in the lung, liver, spine, pancreas, kidney, and prostate. SBRT involves constructing very compact high-dose volumes in and about the tumor. Tumor position must be accurately assessed throughout treatment, especially for tumors that move with respiration. Sophisticated image guidance and related treatment delivery technologies have developed to account for such motion and efficiently deliver high daily dose. All this serves to allow the delivery of ablative dose fractionation to the target capable of both disrupting tumor mitosis and cellular function. Prospective phase I dose-escalation trials have been carried out to reach potent tumoricidal dose levels capable of eradicating tumors with high likelihood. These studies indicate a clear dose-response relationship for tumor control with escalating dose of SBRT. Prospective phase II studies have been reported from several continents consistently showing very high levels of local tumor control. Although late toxicity requires further careful assessment, acute and subacute toxicities are generally acceptable. Patterns of toxicity, both clinical and radiographic, are distinct from those observed with conventionally fractionated radiotherapy as a result of the unique biologic response to ablative fractionation. Prospective trials using SBRT have confirmed the efficacy of treatment in a variety of patient populations. Although mechanisms of ablative-dose injury remain elusive, ongoing prospective trials offer the hope of finding the ideal application for SBRT in the treatment arsenal.
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                Author and article information

                Contributors
                daika99@hiroshima-u.ac.jp
                Journal
                J Appl Clin Med Phys
                J Appl Clin Med Phys
                10.1002/(ISSN)1526-9914
                ACM2
                Journal of Applied Clinical Medical Physics
                John Wiley and Sons Inc. (Hoboken )
                1526-9914
                15 February 2018
                March 2018
                : 19
                : 2 ( doiID: 10.1002/acm2.2018.19.issue-2 )
                : 211-217
                Affiliations
                [ 1 ] Radiation Therapy Section Department of Clinical Support Hiroshima University Hospital Hiroshima Japan
                [ 2 ] Medical and Dental Sciences Course Graduate School of Biomedical & Health Sciences Hiroshima University Hiroshima Japan
                [ 3 ] Department of Radiation Oncology Institute of Biomedical & Health Sciences Hiroshima University Hiroshima Japan
                [ 4 ] Hiroshima High‐Precision Radiotherapy Cancer Center Hiroshima Japan
                [ 5 ] Department of Nuclear Engineering and Management School of Engineering University of Tokyo Tokyo Japan
                [ 6 ] Tokyo Women's Medical University Graduate School of Medicine Medical Physics Tokyo Japan
                Author notes
                [* ] Author to whom correspondence should be addressed. Daisuke Kawahara

                E‐mail: daika99@ 123456hiroshima-u.ac.jp ; Telephone: 81 82 257 5561; Fax: 81 82 257 5561

                Article
                ACM212282
                10.1002/acm2.12282
                5849857
                29450985
                © 2018 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine.

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                Page count
                Figures: 7, Tables: 0, Pages: 7, Words: 3706
                Product
                Categories
                87.55.Gh-
                87.55.k-
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm212282
                March 2018
                Converter:WILEY_ML3GV2_TO_NLMPMC version:version=5.3.2.2 mode:remove_FC converted:14.03.2018

                peg, monte carlo calculation, lipiodol, energy spectrum

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