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      Whole‐body dose equivalent including neutrons is similar for 6 MV and 15 MV IMRT, VMAT, and 3D conformal radiotherapy

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          Abstract

          Purpose

          This study investigates the difference in whole‐body dose equivalent between 6 and 15  MV image‐guided radiotherapy ( IGRT) for the treatment of a rhabdomyosarcoma in the prostate.

          Methods

          A previously developed model for stray radiation of the primary beam was improved and used to calculate the photon dose and photon energy in the out‐of‐field region for a radiotherapy patient. The dose calculated by the treatment planning system was fused with the model‐calculated out‐of‐field dose, resulting in a whole‐body photon dose distribution. The peripheral neutron dose equivalent was calculated using an analytical model from the literature. A daily cone beam CT dose was added to the neutron and photon dose equivalents. The calculated 3D dose distributions were compared to independent measurements conducted with thermoluminescence dosimeters and an anthropomorphic phantom. The dose contributions from the IGRT treatments of three different techniques applied with two nominal X‐ray energies were compared using dose equivalent volume histograms ( DEVHs).

          Results

          The calculated and measured out‐of‐field whole‐body dose equivalents for the IGRT treatments agreed within (9  ± 10) % (mean and type A SD). The neutron dose equivalent was a minor contribution to the total out‐of‐field dose up to 50 cm from the isocenter. Further from the isocenter, head leakage was dominating inside the patient body, whereas the neutron dose equivalent contribution was important close to the surface. There were small differences between the whole‐body DEVHs of the 6 and 15  MV treatments applied with the same technique, although the single scatter contributions showed large differences. Independent of the beam energy, the out‐of‐field dose of the volumetric‐modulated arc therapy ( VMAT) treatment was significantly lower than the dynamic intensity‐modulated radiation therapy ( IMRT) treatment.

          Conclusion

          The calculated whole‐body dose helped to understand the importance of the dose contributions in different areas of the patient. Regarding radiation protection of the patient for IGRT treatments, the choice of beam energy is not important, whereas the treatment technique has a large influence on the out‐of‐field dose. If the patient is treated with intensity‐modulated beams, VMAT should be used instead of dynamic IMRT in terms of radiation protection of the patient. In general, the developed models for photon and neutron dose equivalent calculation can be used for any patient geometry, tumor location, and linear accelerator.

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          Most cited references37

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          Volumetric modulated arc therapy for delivery of prostate radiotherapy: comparison with intensity-modulated radiotherapy and three-dimensional conformal radiotherapy.

          Volumetric modulated arc therapy (VMAT) is a novel form of intensity-modulated radiotherapy (IMRT) optimization that allows the radiation dose to be delivered in a single gantry rotation of up to 360 degrees , using either a constant dose rate (cdr-VMAT) or variable dose rate (vdr-VMAT) during rotation. The goal of this study was to compare VMAT prostate RT plans with three-dimensional conformal RT (3D-CRT) and IMRT plans. The 3D-CRT, five-field IMRT, cdr-VMAT, and vdr-VMAT RT plans were created for 10 computed tomography data sets from patients undergoing RT for prostate cancer. The parameters evaluated included the doses to organs at risk, equivalent uniform doses, dose homogeneity and conformality, and monitor units required for delivery of a 2-Gy fraction. The IMRT and both VMAT techniques resulted in lower doses to normal critical structures than 3D-CRT plans for nearly all dosimetric endpoints analyzed. The lowest doses to organs at risk and most favorable equivalent uniform doses were achieved with vdr-VMAT, which was significantly better than IMRT for the rectal and femoral head dosimetric endpoints (p < 0.05) and significantly better than cdr-VMAT for most bladder and rectal endpoints (p < 0.05). The vdr-VMAT and cdr-VMAT plans required fewer monitor units than did the IMRT plans (relative reduction of 42% and 38%, respectively; p = 0.005) but more than for the 3D-CRT plans (p = 0.005). The IMRT and VMAT techniques achieved highly conformal treatment plans. The vdr-VMAT technique resulted in more favorable dose distributions than the IMRT or cdr-VMAT techniques, and reduced the monitor units required compared with IMRT.
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            The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75.

            Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.
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              The calculated risk of fatal secondary malignancies from intensity-modulated radiation therapy.

              Out-of-field radiation doses to normal tissues may be associated with an increased risk of secondary malignancies, particularly in long-term survivors. Step-and-shoot intensity-modulated radiation therapy (IMRT), an increasingly popular treatment modality, yields higher out-of-field doses than do conventional treatments, because of an increase in required monitor units (beam-on time). We used published risk coefficients (NRCP Report 116) and out-of-field dose equivalents to multiple organ sites to estimate a conservative maximal risk of fatal secondary malignancy associated with 6 IMRT approaches and 1 conventional external-beam approach for prostate cancer. Depending on treatment energy, the IMRT treatments required 3.5-4.9 times as many monitor units to deliver as did the conventional treatment. The conservative maximum risk of fatal second malignancy was 1.7% for conventional radiation, 2.1% for IMRT using 10-MV X-rays, and 5.1% for IMRT using 18-MV X-rays. Intermediate risks were associated with IMRT using 6-MV X-rays: 2.9% for treatment with the Varian accelerator and 3.7% for treatment with the Siemens accelerator, as well as using 15-MV X-rays: 3.4% (Varian) and 4.0% (Siemens). The risk of fatal secondary malignancy differed substantially between IMRT and conventional radiation therapy for prostate cancer, as well as between different IMRT approaches. Perhaps this risk should be considered when choosing the optimal treatment technique and delivery system for patients who will undergo prostate radiation.
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                Author and article information

                Contributors
                pascal.hauri2@uzh.ch
                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
                21 February 2019
                March 2019
                : 20
                : 3 ( doiID: 10.1002/acm2.2019.20.issue-3 )
                : 56-70
                Affiliations
                [ 1 ] Department of Physics University of Zurich Zurich Switzerland
                [ 2 ] Radiotherapy Hirslanden Hirslanden Medical Center Aarau Switzerland
                Author notes
                [*] [* ] Author to whom correspondence should be addressed. Pascal Hauri

                E‐mail: pascal.hauri2@ 123456uzh.ch

                Article
                ACM212543
                10.1002/acm2.12543
                6414138
                30791198
                2b30f795-db2a-4f58-b62d-f0dc442cbd53
                © 2019 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 http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                History
                : 03 October 2018
                : 08 November 2018
                : 31 December 2018
                Page count
                Figures: 9, Tables: 4, Pages: 15, Words: 10635
                Funding
                Funded by: Swiss Cancer League
                Award ID: KFS‐3249‐08‐2013
                Categories
                87.53.Bn
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm212543
                March 2019
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.6.1 mode:remove_FC converted:12.03.2019

                radiotherapy,15 mv vs 6 mv,out‐of‐field dose,neutron dose equivalent

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