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      A benchmarking method to evaluate the accuracy of a commercial proton monte carlo pencil beam scanning treatment planning system

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          Abstract

          Acuros PT is a Monte Carlo algorithm in the Eclipse 13.7 treatment planning system, which is designed to provide rapid and accurate dose calculations for proton therapy. Computational run‐time in minimized by simplifying or eliminating less significant physics processes. In this article, the accuracy of Acuros PT was benchmarked against both measurement and an independent MC calculation, TOPAS. Such a method can be applied to any new MC calculation for the detection of potential inaccuracies. To validate multiple Coulomb scattering ( MCS) which affects primary beam broadening, single spot profiles in a Solidwater ® phantom were compared for beams of five selected proton energies between Acuros PT, measurement and TOPAS. The spot Gaussian sigma in Acuros PT was found to increase faster with depth than both measurement and TOPAS, suggesting that the MCS algorithm in Acuros PT overestimates the scattering effect. To validate Acuros PT modeling of the halo component beyond primary beam broadening, field size factors ( FSF) were compared for multi‐spot profiles measured in a water phantom. The FSF for small field sizes were found to disagree with measurement, with the disagreement increasing with depth. Conversely, TOPAS simulations of the same FSF consistently agreed with measurement to within 1.5%. The disagreement in absolute dose between Acuros PT and measurement was smaller than 2% at the mid‐range depth of multi‐energy beams. While Acuros PT calculates acceptable dose distributions for typical clinical beams, users are cautioned of potentially larger errors at distal depths due to overestimated MCS and halo implementation.

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          Most cited references28

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          Validation of a new grid-based Boltzmann equation solver for dose calculation in radiotherapy with photon beams.

          A new grid-based Boltzmann equation solver, Acuros, was developed specifically for performing accurate and rapid radiotherapy dose calculations. In this study we benchmarked its performance against Monte Carlo for 6 and 18 MV photon beams in heterogeneous media. Acuros solves the coupled Boltzmann transport equations for neutral and charged particles on a locally adaptive Cartesian grid. The Acuros solver is an optimized rewrite of the general purpose Attila software, and for comparable accuracy levels, it is roughly an order of magnitude faster than Attila. Comparisons were made between Monte Carlo (EGSnrc) and Acuros for 6 and 18 MV photon beams impinging on a slab phantom comprising tissue, bone and lung materials. To provide an accurate reference solution, Monte Carlo simulations were run to a tight statistical uncertainty (sigma approximately 0.1%) and fine resolution (1-2 mm). Acuros results were output on a 2 mm cubic voxel grid encompassing the entire phantom. Comparisons were also made for a breast treatment plan on an anthropomorphic phantom. For the slab phantom in regions where the dose exceeded 10% of the maximum dose, agreement between Acuros and Monte Carlo was within 2% of the local dose or 1 mm distance to agreement. For the breast case, agreement was within 2% of local dose or 2 mm distance to agreement in 99.9% of voxels where the dose exceeded 10% of the prescription dose. Elsewhere, in low dose regions, agreement for all cases was within 1% of the maximum dose. Since all Acuros calculations required less than 5 min on a dual-core two-processor workstation, it is efficient enough for routine clinical use. Additionally, since Acuros calculation times are only weakly dependent on the number of beams, Acuros may ideally be suited to arc therapies, where current clinical algorithms may incur long calculation times.
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            Treatment planning and verification of proton therapy using spot scanning: initial experiences.

            Since the end of 1996, we have treated more than 160 patients at PSI using spot-scanned protons. The range of indications treated has been quite wide and includes, in the head region, base-of-skull sarcomas, low-grade gliomas, meningiomas, and para-nasal sinus tumors. In addition, we have treated bone sarcomas in the neck and trunk--mainly in the sacral area--as well as prostate cases and some soft tissue sarcomas. PTV volumes for our treated cases are in the range 20-4500 ml, indicating the flexibility of the spot scanning system for treating lesions of all types and sizes. The number of fields per applied plan ranges from between 1 and 4, with a mean of just under 3 beams per plan, and the number of fluence modulated Bragg peaks delivered per field has ranged from 200 to 45 000. With the current delivery rate of roughly 3000 Bragg peaks per minute, this translates into delivery times per field of between a few seconds to 20-25 min. Bragg peak weight analysis of these spots has shown that over all fields, only about 10% of delivered spots have a weight of more than 10% of the maximum in any given field, indicating that there is some scope for optimizing the number of spots delivered per field. Field specific dosimetry shows that these treatments can be delivered accurately and precisely to within +/-1 mm (1 SD) orthogonal to the field direction and to within 1.5 mm in range. With our current delivery system the mean widths of delivered pencil beams at the Bragg peak is about 8 mm (sigma) for all energies, indicating that this is an area where some improvements can be made. In addition, an analysis of the spot weights and energies of individual Bragg peaks shows a relatively broad spread of low and high weighted Bragg peaks over all energy steps, indicating that there is at best only a limited relationship between pencil beam weighting and depth of penetration. This latter observation may have some consequences when considering strategies for fast re-scanning on second generation scanning gantries.
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              Experimental characterization and physical modelling of the dose distribution of scanned proton pencil beams.

              In this paper we present the pencil beam dose model used for treatment planning at the PSI proton gantry, the only system presently applying proton therapy with a beam scanning technique. The scope of the paper is to give a general overview on the various components of the dose model, on the related measurements and on the practical parametrization of the results. The physical model estimates from first physical principles absolute dose normalized to the number of incident protons. The proton beam flux is measured in practice by plane-parallel ionization chambers (ICs) normalized to protons via Faraday-cup measurements. It is therefore possible to predict and deliver absolute dose directly from this model without other means. The dose predicted in this way agrees very well with the results obtained with ICs calibrated in a cobalt beam. Emphasis is given in this paper to the characterization of nuclear interaction effects, which play a significant role in the model and are the major source of uncertainty in the direct estimation of the absolute dose. Nuclear interactions attenuate the primary proton flux, they modify the shape of the depth-dose curve and produce a faint beam halo of secondary dose around the primary proton pencil beam in water. A very simple beam halo model has been developed and used at PSI to eliminate the systematic dependences of the dose observed as a function of the size of the target volume. We show typical results for the relative (using a CCD system) and absolute (using calibrated ICs) dosimetry, routinely applied for the verification of patient plans. With the dose model including the nuclear beam halo we can predict quite precisely the dose directly from treatment planning without renormalization measurements, independently of the dose, shape and size of the dose fields. This applies also to the complex non-homogeneous dose distributions required for the delivery of range-intensity-modulated proton therapy, a novel therapy technique developed at PSI.
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                Author and article information

                Contributors
                linl@uphs.upenn.edu
                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
                02 February 2017
                March 2017
                : 18
                : 2 ( doiID: 10.1002/acm2.2017.18.issue-2 )
                : 44-49
                Affiliations
                [ 1 ] Department of Radiation Oncology University of Pennsylvania Philadelphia Pennsylvania USA
                [ 2 ] Varian Medical Systems Palo Alto California USA
                Author notes
                [*] [* ]Author to whom correspondence should be addressed. Liyong Lin E‐mail: linl@ 123456uphs.upenn.edu .
                Article
                ACM212043
                10.1002/acm2.12043
                5689961
                28300385
                239170ed-3a01-4238-bc18-45624e9b1968
                © 2017 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.

                History
                : 23 October 2016
                : 16 December 2016
                Page count
                Figures: 4, Tables: 1, Pages: 6, Words: 3839
                Categories
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm212043
                March 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:16.11.2017

                acurospt,commissioning,monte carlo dose calculation,pencil beam scanning,proton therapy

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