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      Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

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          Abstract

          Purpose

          This study investigates a large‐area plane‐parallel ionization chamber ( LAC) for measurements of dose‐area product in water ( DAP w) in megavoltage ( MV) photon fields.

          Methods

          Uniformity of electrode separation of the LAC ( PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high‐resolution micro CT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm 2 field were acquired. A 6 MV calibration coefficient, N D ,w, LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP w. Additional calibrations in 10 × 10 cm 2 fields at 6, 10, and 18 MV were performed.

          Results

          Electrode separation is uniform and agrees with specifications. Volume‐averaging leads to a signal increase proportional to ~1/cos( α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off‐axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm 2 field. N D ,w, LAC was 163.7 mGy cm −2 nC −1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm 2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.

          Conclusions

          The LAC requires only small correction factors for DAP w measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm 2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over‐response. The accuracy of relative point‐dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP w measurements.

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          Most cited references 22

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          Multichannel film dosimetry with nonuniformity correction.

          A new method to evaluate radiochromic film dosimetry data scanned in multiple color channels is presented. This work was undertaken to demonstrate that the multichannel method is fundamentally superior to the traditional single channel method. The multichannel method allows for the separation and removal of the nondose-dependent portions of a film image leaving a residual image that is dependent only on absorbed dose. Radiochromic films were exposed to 10 x 10 cm radiation fields (Co-60 and 6 MV) at doses up to about 300 cGy. The films were scanned in red-blue-green (RGB) format on a flatbed color scanner and measured to build calibration tables relating the absorbed dose to the response of the film in each of the color channels. Film images were converted to dose maps using two methods. The first method used the response from a single color channel and the second method used the response from all three color channels. The multichannel method allows for the separation of the scanned signal into one part that is dose-dependent and another part that is dose-independent and enables the correction of a variety of disturbances in the digitized image including nonuniformities in the active coating on the radiochromic film as well as scanner related artifacts. The fundamental mathematics of the two methods is described and the dose maps calculated from film images using the two methods are compared and analyzed. The multichannel dosimetry method was shown to be an effective way to separate out non-dose-dependent abnormalities from radiochromic dosimetry film images. The process was shown to remove disturbances in the scanned images caused by nonhomogeneity of the radiochromic film and artifacts caused by the scanner and to improve the integrity of the dose information. Multichannel dosimetry also reduces random noise in the dose images and mitigates scanner-related artifacts such as lateral position dependence. In providing an ability to calculate dose maps from data in all the color channels the multichannel method provides the ability to examine the agreement between the color channels. Furthermore, when using calibration data to convert RGB film images to dose using the new method, poor correspondence between the dose calculations for the three color channels provides an important indication that the this new technique enables easy indication in case the dose and calibration films are curve mismatched. The method permit compensation for thickness nonuniformities in the film, increases the signal to noise level, mitigates the lateral dose-dependency of flatbed scanners effect of the calculated dose map and extends the evaluable dose range to 10 cGy-100 Gy. Multichannel dosimetry with radiochromic film like Gafchromic EBT2 is shown to have significant advantages over single channel dosimetry. It is recommended that the dosimetry protocols described be implemented when using this radiochromic film to ensure the best data integrity and dosimetric accuracy.
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            Detector comparison for small field output factor measurements in flattening filter free photon beams.

            The applicability of various detectors for small field dosimetry and whether there are differences in the detector response when irradiated with FF- and FFF-beams was investigated. Output factors of 6 and 10 MV FF- and FFF-beams were measured with 14 different online detectors using field sizes between 10 × 10 and 0.6 × 0.6 cm(2) at a depth of 5 cm of water in isocentric conditions. Alanine pellets with a diameter of 5 and 2.5mm were used as reference dosimeters for field sizes down to 1.2 × 1.2 and 0.6 × 0.6 cm(2), respectively. The ratio of the relative output measured with the online detectors to the relative output measured with alanine was evaluated (referred to as dose response ratio). The dose response ratios of two different shielded diodes measured with 10 MV FF-beams deviated substantially by 2-3% compared to FFF-beams at a field size of 0.6 × 0.6 cm(2). This difference was less pronounced for 6 MV FF- and FFF-beams. For all other detectors the dose response ratios of FF- and FFF-beams showed no significant difference. The dose response ratios of the majority of the detectors agreed within the measurement uncertainty when irradiated with FF- and FFF-beams. Of all investigated detectors, the microDiamond and the unshielded diodes would require only small corrections which make them suitable candidates for small field dosimetry in FF- and FFF-beams. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.
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              A Monte Carlo model for calculating out-of-field dose from a varian 6 MV beam.

              Dose to the patient outside of the treatment field is important when evaluating the outcome of radiotherapy treatments. However, determining out-of-field doses for any particular treatment plan currently requires either time-consuming measurements or calculated estimations that may be highly uncertain. A Monte Carlo model may allow these doses to be determined quickly, accurately, and with a great degree of flexibility. MCNPX was used to create a Monte Carlo model of a Varian Clinac 2100 accelerator head operated at 6 MV. Simulations of the dose out-of-field were made and measurements were taken with thermoluminescent dosimeters in an acrylic phantom and with an ion chamber in a water tank to validate the Monte Carlo model. Although local differences between the out-of-field doses calculated by the model and those measured did exceed 50% at some points far from the treatment field, the average local difference was only 16%. This included a range of doses as low as 0.01% of the central axis dose, and at distances in excess of 50 cm from the central axis of the treatment field. The out-of-field dose was found to vary with field size and distance from the central axis, but was almost independent of the depth in the phantom except where the dose increased substantially at depths less than dmax. The relationship between dose and kerma was also investigated, and kerma was found to be a good estimate of dose (within 3% on average) except near the surface and in the field penumbra. Our Monte Carlo model was found to well represent typical Varian 2100 accelerators operated at 6 MV.
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                Author and article information

                Contributors
                tom.kupfer@austin.org.au
                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
                04 October 2017
                November 2017
                : 18
                : 6 ( doiID: 10.1002/acm2.2017.18.issue-6 )
                : 206-217
                Affiliations
                [ 1 ] School of Science RMIT University Melbourne Vic. Australia
                [ 2 ] Radiation Oncology Centre Austin Health Heidelberg Vic. Australia
                [ 3 ] School of Science RMIT University Melbourne Vic. Australia
                [ 4 ] Faculty of Science The University of Sydney Sydney NSW Australia
                [ 5 ] Department of Radiation Oncology Calvary Mater Newcastle Waratah NSW Australia
                [ 6 ] Australian Radiation Protection and Nuclear Safety Agency Yallambie Vic. Australia
                [ 7 ] School of Science RMIT University Melbourne Vic. Australia
                Author notes
                [* ] Author to whom correspondence should be addressed. Tom Kupfer

                E‐mail: tom.kupfer@ 123456austin.org.au

                Article
                ACM212185
                10.1002/acm2.12185
                5689907
                28980432
                © 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.

                Page count
                Figures: 7, Tables: 5, Pages: 12, Words: 10580
                Product
                Categories
                87.53.Bn
                87.55.Qr
                29.40.Cs
                41.50.th
                06.20.fb
                Radiation Measurements
                Radiation Measurements
                Custom metadata
                2.0
                acm212185
                November 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:16.11.2017

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