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      Relative stopping power measurements to aid in the design of anthropomorphic phantoms for proton radiotherapy

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          Abstract

          The delivery of accurate proton dose for clinical trials requires that the appropriate conversion function from Hounsfield unit (HU) to relative linear stopping power (RLSP) be used in proton treatment planning systems (TPS). One way of verifying that the TPS is calculating the correct dose is an end‐to‐end test using an anthropomorphic phantom containing tissue‐equivalent materials and dosimeters. Many of the phantoms in use for such end‐to‐end tests were originally designed using tissue‐equivalent materials that had physical characteristics to match patient tissues when irradiated with megavoltage photon beams. The aim of this study was to measure the RLSP of materials used in the phantoms, as well as alternative materials to enable modifying phantoms for use at proton therapy centers. Samples of materials used and projected for use in the phantoms were measured and compared to the HU assigned by the treatment planning system. A percent difference in RLSP of 5% was used as the cutoff for materials deemed acceptable for use in proton therapy (i.e., proton equivalent). Until proper tissue‐substitute materials are identified and incorporated, institutions that conduct end‐to‐end tests with the phantoms are instructed to override the TPS with the measured stopping powers we provide. To date, the RLSPs of 18 materials have been measured using a water phantom and/or multilayer ion chamber (MLIC). Nine materials were identified as acceptable for use in anthropomorphic phantoms. Some of the failing tissue substitute materials are still used in the current phantoms. Further investigation for additional appropriate tissue substitute materials in proton beams is ongoing. Until all anthropomorphic phantoms are constructed of appropriate materials, a unique HU‐RLSP phantom has been developed to be used during site visits to verify the proton facility's treatment planning HU‐RLSP calibration curve.

          PACS number: 87.53.Bn

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          The calibration of CT Hounsfield units for radiotherapy treatment planning.

          Computer tomographic (CT) scans are used to correct for tissue inhomogeneities in radiotherapy treatment planning. In order to guarantee a precise treatment, it is important to obtain the relationship between CT Hounsfield units and electron densities (or proton stopping powers for proton radiotherapy), which is the basic input for radiotherapy planning systems which consider tissue heterogeneities. A method is described to determine improved CT calibrations for biological tissue (a stoichiometric calibration) based on measurements using tissue equivalent materials. The precision of this stoichiometric calibration and the more usual tissue substitute calibration is determined by a comparison of calculated proton radiographic images based on these calibrations and measured radiographs of a biological sample. It has been found that the stoichiometric calibration is more precise than the tissue substitute calibration.
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            Challenges in credentialing institutions and participants in advanced technology multi-institutional clinical trials.

            The Radiological Physics Center (RPC) has functioned continuously for 38 years to assure the National Cancer Institute and the cooperative groups that institutions participating in multi-institutional trials can be expected to deliver radiation treatments that are clinically comparable to those delivered by other institutions in the cooperative groups. To accomplish this, the RPC monitors the machine output, the dosimetry data used by the institutions, the calculation algorithms used for treatment planning, and the institutions' quality control procedures. The methods of monitoring include on-site dosimetry review by an RPC physicist and a variety of remote audit tools. The introduction of advanced technology clinical trials has prompted several study groups to require participating institutions and personnel to become credentialed, to ensure their familiarity and capability with techniques such as three-dimensional conformal radiotherapy, intensity-modulated radiotherapy, stereotactic body radiotherapy, and brachytherapy. The RPC conducts a variety of credentialing activities, beginning with questionnaires to evaluate an institution's understanding of the protocol and their capabilities. Treatment-planning benchmarks are used to allow the institution to demonstrate their planning ability and to facilitate a review of the accuracy of treatment-planning systems under relevant conditions. The RPC also provides mailable anthropomorphic phantoms to verify tumor dose delivery for special treatment techniques. While conducting these reviews, the RPC has amassed a large amount of data describing the dosimetry at participating institutions. Representative data from the monitoring programs are discussed, and examples are presented of specific instances in which the RPC contributed to the discovery and resolution of dosimetry errors.
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              Design, development, and implementation of the radiological physics center's pelvis and thorax anthropomorphic quality assurance phantoms.

              The Radiological Physics Center (RPC) developed two heterogeneous anthropomorphic quality assurance phantoms for use in verifying the accuracy of radiation delivery: one for intensity-modulated radiation therapy (IMRT) to the pelvis and the other for stereotactic body radiation therapy (SBRT) to the thorax. The purpose of this study was to describe the design and development of these two phantoms and to demonstrate the reproducibility of measurements generated with them. The phantoms were built to simulate actual patient anatomy. They are lightweight and water-fillable, and they contain imageable targets and organs at risk of radiation exposure that are of similar densities to their human counterparts. Dosimetry inserts accommodate radiochromic film for relative dosimetry and thermoluminesent dosimetry capsules for absolute dosimetry. As a part of the commissioning process, each phantom was imaged, treatment plans were developed, and radiation was delivered at least three times. Under these controlled irradiation conditions, the reproducibility of dose delivery to the target TLD in the pelvis and thorax phantoms was 3% and 0.5%, respectively. The reproducibility of radiation-field localization was less than 2.5 mm for both phantoms. Using these anthropomorphic phantoms, pelvic IMRT and thoracic SBRT radiation treatments can be verified with a high level of precision. These phantoms can be used to effectively credential institutions for participation in specific NCI-sponsored clinical trials.
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                Author and article information

                Contributors
                Ryan.Grant@mdanderson.org
                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
                06 March 2014
                March 2014
                : 15
                : 2 ( doiID: 10.1002/acm2.2014.15.issue-2 )
                : 121-126
                Affiliations
                [ 1 ] The Graduate School of Biomedical Sciences The University of Texas Health Science Center Houston TX
                [ 2 ] Department of Radiation Physics The University of Texas M.D. Anderson Cancer Center Houston TX
                [ 3 ] Office of Medical Physics and Radiation Safety Boston University Medical Center Boston MA USA
                Author notes
                [*] [* ] a Corresponding author: Ryan L. Grant, Radiation Physics, MD Anderson Cancer Center, 1515 Holcombe Blvd Unit 607, Houston, TX 77030, USA; phone: (713) 745 8989; fax: (713) 794 1364; email: Ryan.Grant@ 123456mdanderson.org

                Article
                ACM20121
                10.1120/jacmp.v15i2.4523
                4283476
                24710437
                9f6a6de5-fff1-458f-83cc-9bdb5db7cf9f
                © 2014 The Authors.

                This is an open access article under the terms of the http://creativecommons.org/licenses/by/3.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                History
                : 05 May 2013
                : 06 November 2013
                Page count
                Figures: 3, Tables: 1, References: 8, Pages: 6, Words: 2880
                Funding
                Funded by: Massachusetts General Hospital
                Award ID: C06 CA059267
                Award ID: CA10953
                Award ID: CA81647
                Categories
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm20121
                March 2014
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.6.1 mode:remove_FC converted:14.03.2019

                quality assurance,proton therapy,stopping powers
                quality assurance, proton therapy, stopping powers

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