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      Photon and electron backscatter dose and energy spectrum analysis around Lipiodol using flattened and unflattened beams

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          Abstract

          Purpose

          The aim of the current study was to evaluate the backscatter dose and energy spectrum from the Lipiodol with flattening filter (FF) and flattening filter‐free (FFF) beams. Moreover, the backscatter range, that was defined as the backscatter distance (BD) are revealed.

          Methods

          6 MVX FF and FFF beams were delivered by TrueBeam. Two dose calculation methods with Monte Carlo calculation were used with a virtual phantom in which the Lipiodol (3 × 3 × 3 cm 3) was located at a depth of 5.0 cm in a water‐equivalent phantom (20 × 20 × 20 cm 3). The first dose calculation was an analysis of the dose and energy spectrum with the complete scattering of photons and electrons, and the other was a specified dose analysis only with scattering from a specified region. The specified dose analysis was divided into a scattering of primary photons and a scattering of electrons.

          Results

          The lower‐energy photons contributed to the backscatter, while the high‐energy photons contributed the difference of the backscatter dose between the FF and FFF beams. Although the difference in the dose from the scattered electrons between the FF and FFF beams was within 1%, the difference of the dose from the scattered photons between the FF and FFF beams was 5.4% at a depth of 4.98 cm.

          Conclusions

          The backscatter range from the Lipiodol was within 3 mm and depended on the Compton scatter from the primary photons. The backscatter dose from the Lipiodol can be useful in clinical applications in cases where the backscatter region is located within a tumor.

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          Most cited references 18

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          Delivery time comparison for intensity-modulated radiation therapy with/without flattening filter: a planning study.

          The treatment delivery time of intensity-modulated radiation therapy (IMRT) with a multileaf collimator (MLC) is generally longer than that of conventional radiotherapy. In theory, removing the flattening filter from the treatment head may reduce the beam-on time by enhancing the output dose rate, and then reduce the treatment delivery time. And in practice, there is a possibility of delivering the required fluence distribution by modulating the unflattened non-uniform fluence distribution. However, the reduction of beam-on time may be discounted by the increase of leaf-travel time and (or) verification-and-recording (V&R) time. Here we investigate the overall effect of flattening filter on the treatment delivery time of IMRT with MLCs implemented in the step and shoot method, as well as with compensators on six hybrid machines. We compared the treatment delivery time with/without flattening filter for ten nasopharynx cases and ten prostate cases by observing the variations of the ratio of the beam-on time, segment number, leaf-travel time and the treatment delivery time with dose rate, leaf speed and V&R time. The results show that, without the flattening filter, the beam-on time reduces for both static MLC and compensator-based techniques: the number of segments and the leaf-travel time increase slightly for the static MLC technique; the relative IMRT treatment delivery time decreases more with lower dose rate, higher leaf speed and shorter V&R overhead time. The absolute treatment delivery time reduction depends on the fraction dose. It is not clinically significant at a fraction dose of 2 Gy for the technique of removing the flattening filter, but becomes significant when the fraction dose is as high as that for radiosurgery.
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            Radiotherapy after transcatheter arterial chemoembolization for patients with hepatocellular carcinoma and portal vein tumor thrombus.

            Transcatheter arterial chemoembolization (TACE) is used in the treatment of hepatocellular carcinoma; however, it has limited effect on portal vein tumor thrombus (PVTT). The purpose of this study was to assess the feasibility and efficacy of radiotherapy targeting the PVTT after TACE for the tumor in the hepatic parenchyma. TACE was performed using epirubicin hydrochloride, iodized poppy seed oil, and gelatin sponge particles. Radiotherapy was performed targeting the PVTT to a total dose of 50 Gy in 25 fractions during 5 weeks. Twenty consecutive patients were treated with this combined treatment. Sixteen of 20 patients could complete the planned radiotherapy. Partial response was observed in 10, no change in 4, and progression in 6. The response rate was 50% (95% CI 28-72%). The 1-year overall survival rate was 25% (95% CI 6-44%), and the median survival time was 5.3 months. It was difficult to determine the late toxicities because of disease progression and additional TACE, and only one patient died without disease progression. Radiotherapy after TACE is feasible for patients with hepatocellular carcinoma and PVTT. The survival figure, however, is still dismal, and further investigation is needed to establish the best combination of treatment modalities.
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              Accuracy of daily image guidance for hypofractionated liver radiotherapy with active breathing control.

              A six-fraction, high-precision radiotherapy protocol for unresectable liver cancer has been developed in which active breathing control (ABC) is used to immobilize the liver and daily megavoltage (MV) imaging and repositioning is used to decrease geometric uncertainties. We report the accuracy of setup in the first 20 patients consecutively treated using this approach. After setup using conventional skin marks and lasers, orthogonal MV images were acquired with the liver immobilized using ABC. The images were aligned to reference digitally reconstructed radiographs using the diaphragm for craniocaudal (CC) alignment and the vertebral bodies for anterior-posterior (AP) and mediolateral (ML) alignment. Adjustments were made for positioning errors >3 mm. Verification imaging was repeated after repositioning to assess for residual positioning error. Offline image matching was conducted to determine the setup accuracy using this approach compared with the initial setup error before repositioning. Real-time beam's-eye-view MV movies containing an air-diaphragm interface were also evaluated. A total of 405 images were evaluated from 20 patients. Repositioning occurred in 109 of 120 fractions because of offsets >3 mm. Three to eight beam angles, with up to four segments per field, were used for each isocenter. Breath holds of up to 27 s were used for imaging and treatment. The average time from the initial verification image to the last treatment beam was 21 min. Image guidance and repositioning reduced the population random setup errors (sigma) from 6.5 mm (CC), 4.2 mm (ML), and 4.7 mm (AP) to 2.5 mm (CC), 2.8 mm (ML), and 2.9 mm (AP). The average individual random setup errors (sigma) were reduced from 4.5 mm (CC), 3.2 mm (AP), and 2.5 mm (ML) to 2.2 mm (CC), 2.0 mm (AP), and 2.0 mm (ML). The standard deviation of the distribution of systematic deviations (Sigma) was also reduced from 5.1 mm (CC), 3.4 mm (ML), and 3.1 mm (AP) to 1.4 mm (CC), 2.0 mm (ML), and 1.9 mm (AP) with image guidance and repositioning. The average absolute systematic errors were reduced from 4.1 mm (CC), 2.4 mm (AP), and 3.1 (ML) to 1.1 mm (CC), 1.3 mm (AP), and 1.6 mm (ML). Analysis of 52 real-time beam's-eye-view MV movies revealed an average absolute CC offset in diaphragm position of 1.9 mm. Image guidance with orthogonal MV imaging and ABC for stereotactic body radiotherapy for liver cancer is feasible, improving setup accuracy compared with ABC without daily imaging and repositioning.
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                Author and article information

                Contributors
                daika99@hiroshima-u.ac.jp
                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
                18 March 2019
                June 2019
                : 20
                : 6 ( doiID: 10.1002/acm2.2019.20.issue-6 )
                : 178-183
                Affiliations
                [ 1 ] Radiation Therapy Section Department of Clinical Support Hiroshima University Hospital Hiroshima Japan
                [ 2 ] Department of Radiation Oncology Institute of Biomedical & Health Sciences Hiroshima University Hiroshima Japan
                [ 3 ] Department of Radiation Oncology Graduate School of Medicine Yamaguchi University Yamaguchi Japan
                Author notes
                [* ] Author to whom correspondence should be addressed. Daisuke Kawahara

                E‐mail: daika99@ 123456hiroshima-u.ac.jp ; Telephone: +81‐82‐257‐1545.

                Article
                ACM212560
                10.1002/acm2.12560
                6560232
                30884060
                © 2019 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 http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                Page count
                Figures: 6, Tables: 0, Pages: 6, Words: 3433
                Product
                Categories
                61.80.−x
                Radiation Measurements
                Radiation Measurements
                Custom metadata
                2.0
                acm212560
                June 2019
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.6.4 mode:remove_FC converted:12.06.2019

                lipiodol, dose enhancement, backscatter

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