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      Investigating treatment dose error due to beam attenuation by a carbon fiber tabletop

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          Abstract

          Carbon fiber is commonly used in radiation therapy for treatment tabletops and various immobilization and support devices, partially because it is generally perceived to be almost radiotransparent to high‐energy photons. To avoid exposure to normal tissue during modern radiation therapy, one must deliver the radiation from all gantry angles; hence, beams often transit the couch proximal to the patient. The effects of the beam attenuation by the support structure of the couch are often neglected in the planning process. In this study, we investigate the attenuation of 6‐MV and 18‐MV photon beams by a Medtec (Orange City, IA) carbon fiber couch. We have determined that neglecting the attenuation of oblique treatment fields by the carbon fiber couch can result in localized dose reduction from 4% to 16%, depending on energy, field size, and geometry. Further, we investigate the ability of a commercial treatment‐planning system (Theraplan Plus v3.8) to account for the attenuation by the treatment couch. Results show that incorporating the carbon fiber couch in the patient model reduces the dose error to less than 2%. The variation in dose reduction as a function of longitudinal couch position was also measured. In the triangular strut region of the couch, the attenuation varied ± 0.5 % following the periodic nature of the support structure. Based on these findings, we propose the routine incorporation of the treatment tabletop into patient treatment planning dose calculations.

          PACS numbers: 87.53.Dq, 87.53.Mr

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          Most cited references13

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          What degree of accuracy is required and can be achieved in photon and neutron therapy?

          In this paper an attempt is made to formulate criteria for the accuracy in the delivery of absorbed dose to a patient during photon or neutron therapy. These requirements are mainly based on the relative steepness of dose-effect curves for local tumour control and normal tissue damage. A review of these dose-effect curves after photon irradiation shows a great variety in steepness; the curves for normal tissue complications in general may be steeper than those for local tumour control. From these data a standard requirement for the combined uncertainty of type A (random) and type B (systematic), given as one relative standard deviation, in the absorbed dose delivery of 3.5% is proposed, even though it is known that in many cases larger values are acceptable and in a few special cases an even smaller value should be aimed at. From the available radiobiological and clinical data it can be concluded that no statistically significant difference can be observed in the relative steepness of dose-effect curves after photon or neutron irradiation. Similar limits will thus be requested in neutron therapy. The uncertainties in the various steps involved in the delivery of an absorbed dose to a point in a patient have been analysed for a treatment with two parallel-opposed beams. The results of this analysis showed that even for these simple treatment conditions, the required accuracy in the delivery of the absorbed dose cannot completely be obtained in photon therapy, and not nearly in neutron therapy. The uncertainties in physical, radiobiological and clinical approaches for weighting of the biological effectiveness of neutron radiation have been compared. The uncertainty in the RBE ratio will replace the type B uncertainty in the absorbed dose during patient treatment if the same dosimetry protocol is applied during biological and clinical procedures.
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            The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert.

            The use of rigid carbon fiber couch inserts in radiotherapy treatment couches is a well-established method of reducing patient set-up errors associated with couch sag. Several published studies have described such inserts as radiotranslucent with negligible attenuation of the radiation field. Most of these studies were conducted with the radiation field normally incident on the couch and there appears to be no evidence in the literature of the effect of the gantry angle on the extent of beam attenuation by the carbon fiber insert alone during external beam radiotherapy. In this study we examined the magnitude of this effect over a range of posterior oblique gantry angles using a cylindrical solid water phantom containing an ionization chamber placed isocentrically. It was found that a 6 MV photon beam, field size 10 x 5 cm, was attenuated significantly as the gantry angle approached the plane of the couch, from 2% at normal incidence and reaching 9% attenuation at angle of incidence 70 degrees. This could have serious implications regarding dose to the treatment volume for treatments requiring posterior oblique angles of incidence with a possible correction factor necessary in monitor unit calculations.
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              The effect of carbon fibre inserts on the build-up and attenuation of high energy photon beams.

              The use of new materials in radiotherapy requires an investigation of the effects of these materials on the relevant beam parameters. The high strength and low density of carbon fibre suggest an excellent material for table inserts with minimal attenuation, without changing the skin sparing effect in the build-up zone. In this paper three different carbon fibre plates and two conventionally table top materials are studied in Co-60, 6 MV and 23 MV photon beams. From depth dose measurements it is clear that the dose in the build-up zone is influenced in the qualities of the beams. The mutual differences for the three carbon plates are minimal. For Co-60 the depth of the maximum dose is decreased by carbon from 5 to 2 mm and the surface dose is increased from 18 to 76%. For 6 and 23 MV the surface dose is increased from 21 to 52% and 20 to 32%, respectively, as well as the dose in the build-up region. A transmission of 99% was measured for two carbon plates out of three in Co-60 and for one out of three in 6 MV.
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                Author and article information

                Contributors
                kmyint@ottawahospital.on.ca
                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
                24 August 2006
                Summer 2006
                : 7
                : 3 ( doiID: 10.1002/acm2.2006.7.issue-3 )
                : 21-27
                Affiliations
                [ 1 ] Ottawa Hospital Regional Cancer Centre Ottawa Ontario Canada
                Author notes
                [*] [* ]Corresponding author: Kenji Myint, Department of Medical Physics, Ottawa Hospital Regional Cancer Centre, 503 Smyth Rd., Ottawa, ON K1H 8L6 Canada; email: kmyint@ 123456ottawahospital.on.ca
                Article
                ACM20021
                10.1120/jacmp.v7i3.2247
                5722426
                17533341
                caec9f26-9217-48f9-bb10-3802bcfd4265
                © 2006 The Authors.

                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
                : 10 February 2006
                : 13 June 2006
                Page count
                Figures: 4, Tables: 2, References: 13, Pages: 7, Words: 2907
                Funding
                Funded by: Ottawa Regional Cancer Centre Foundation
                Categories
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm20021
                Summer 2006
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:17.11.2017

                radiotherapy,couch attenuation,dose error
                radiotherapy, couch attenuation, dose error

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