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      Absorbed dose and dose rate using the Varian OBI 1.3 and 1.4 CBCT system

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          Abstract

          According to published data, the absorbed dose used for a CBCT image acquisition with Varian OBI v1.3 can be as high as 100 mGy. In 2008 Varian released a new OBI version (v1.4), which promised to reduce the imaging dose. In this study, absorbed doses used for CBCT image acquisitions with the default irradiation techniques of Varian OBI v1.3 and v1.4 are measured.

          TLDs are used to derive dose distributions at three planes inside an anthropomorphic phantom. In addition, point doses and dose profiles inside a ‘stack’ of three CTDI body phantoms are measured using a new solid state detector, the CT Dose Profiler. With the CT Dose Profiler, the individual pulses from the X‐ray tube are also studied. To verify the absorbed dose measured with the CT Dose Profiler, it is compared to TLD. The image quality is evaluated using a Catphan phantom.

          For OBI v1.3, doses measured in transverse planes of the Alderson phantom range between 64 mGy and 144 mGy. The average dose is around 100 mGy. For OBI v1.4, doses measured in transverse planes of the Alderson phantom range between 1 mGy and 51 mGy. Mean doses range between 3‐35 mGy depending on CBCT mode. CT Dose Profiler data agree with TLD measurements in a CTDI phantom within the uncertainty of the TLD measurements (estimated SD ± 10 % ). Instantaneous dose rate at the periphery of the phantom can be higher than 20 mGy/s, which is 10 times the dose rate at the center. The spatial resolution in v1.4 is not as high as in v1.3.

          In conclusion, measurements show that the imaging doses for default modes in Varian OBI v1.4 CBCT system are significantly lower than in v1.3. The CT Dose Profiler is proven fast and accurate for CBCT applications.

          PACS number: 87.53.Bn

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          Most cited references16

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          A dose comparison study between XVI and OBI CBCT systems.

          The purpose of this study is to establish a comprehensive set of dose measurements data obtained from the X-ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On-Board Imager (OBI, Varian Medical Systems) cone-beam CT (CBCT) systems. To this end, two uniform-density cylindrical acrylic phantoms with diameters of 18 cm (head phantom) and 30 cm (body phantom) were used for all measurements. Both phantoms included ion chamber placement holes in the center and at periphery (2 cm below surface). For the XVI unit, the four standard manufacturer-supplied protocols were measured. For the OBI unit, the full bow tie and half bow tie (and no bow tie) filters were used in combination with the two scanning modes; namely, full-fan and half-fan. The total milliampere x seconds (mA s) setting was also varied for each protocol to establish the linear relationship between the dose deposited and the mA s used (with all other factors being held constant). Half-value layers in aluminum (Al) were also measured for beam characteristic determination. For the XVI unit, the average dose ranged from 0.1 to 3.5 cGy with the highest dose measured using the "prostate" protocol with the body phantom. For the OBI unit, the average dose ranged from 1.1 to 8.3 cGy with the highest dose measured using the full-fan protocol with the head phantom. The measured doses were highly linear as a function of mA s, for both units, where the measurement points followed a linear relationship very closely with R2 > 0.99 for all cases. Half-value layers were between 4.6- and 7.0-mm-Al for the two CBCT units where XVI generally had more penetrating beams at the similar kVp settings. In conclusion, a comprehensive series of dose measurements were performed on the XVI and the OBI CBCT units. In the process, many of the important similarities and differences between the two systems were observed and summarized in this work.
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            Accurate patient dosimetry of kilovoltage cone-beam CT in radiation therapy.

            The increased utilization of x-ray imaging in image-guided radiotherapy has dramatically improved the radiation treatment and the lives of cancer patients. Daily imaging procedures, such as cone-beam computed tomography (CBCT), for patient setup may significantly increase the dose to the patient's normal tissues. This study investigates the dosimetry from a kilovoltage (kV) CBCT for real patient geometries. Monte Carlo simulations were used to study the kV beams from a Varian on-board imager integrated into the Trilogy accelerator. The Monte Carlo calculated results were benchmarked against measurements and good agreement was obtained. The authors developed a novel method to calibrate Monte Carlo simulated beams with measurements using an ionization chamber in which the air-kerma calibration factors are obtained from an Accredited Dosimetry Calibration Laboratory. The authors have introduced a new Monte Carlo calibration factor, fMCcal, which is determined from the calibration procedure. The accuracy of the new method was validated by experiment. When a Monte Carlo simulated beam has been calibrated, the simulated beam can be used to accurately predict absolute dose distributions in the irradiated media. Using this method the authors calculated dose distributions to patient anatomies from a typical CBCT acquisition for different treatment sites, such as head and neck, lung, and pelvis. Their results have shown that, from a typical head and neck CBCT, doses to soft tissues, such as eye, spinal cord, and brain can be up to 8, 6, and 5 cGy, respectively. The dose to the bone, due to the photoelectric effect, can be as much as 25 cGy, about three times the dose to the soft tissue. The study provides detailed information on the additional doses to the normal tissues of a patient from a typical kV CBCT acquisition. The methodology of the Monte Carlo beam calibration developed and introduced in this study allows the user to calculate both relative and absolute absorbed doses.
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              A new look at CT dose measurement: Beyond CTDI

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                Author and article information

                Contributors
                asa.palm@vgregion.se
                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
                28 January 2010
                Winter 2010
                : 11
                : 1 ( doiID: 10.1002/acm2.2010.11.issue-1 )
                : 229-240
                Affiliations
                [ 1 ] Sahlgrenska University Hospital Deptartment of Medical Physics and Biomedical Engineering Göteborg Sweden
                [ 2 ] RTI Electronics AB Mölndal Sweden
                Author notes
                [*] [* ]Corresponding author: Åsa Palm, Sahlgrenska University Hospital. Deptartment of Medical Physics and Biomedical Engineering, SE‐413 45 Göteborg, Sweden; phone: +46 31 3427238; fax: +46 31 412037; email: asa.palm@ 123456vgregion.se
                Article
                ACM20229
                10.1120/jacmp.v11i1.3085
                5719770
                20160695
                5fea4dbf-6c6c-4e1d-ab18-9180bb835c26
                © 2010 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
                : 05 May 2009
                : 22 October 2009
                Page count
                Figures: 8, Tables: 7, References: 19, Pages: 12, Words: 4493
                Categories
                Radiation Measurements
                Radiation Measurements
                Custom metadata
                2.0
                acm20229
                Winter 2010
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:16.11.2017

                cone‐beam ct,absorbed dose,tld,ct dose profiler
                cone‐beam ct, absorbed dose, tld, ct dose profiler

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