Abstract
We developed a novel low‐radiation‐absorbent lok‐bar (HM‐bar) that is used to secure the immobilizers to the couch. The aim of this study was to investigate the X‐ray scattering and absorption properties of the HM‐bar in computed tomography (CT) simulation and radiotherapy dose delivery using the Varian Exact™ lok‐bar (VL‐bar) as a benchmark. CT images were obtained with or without lok‐bar, and then each image was visually evaluated for artifacts. The attenuation rates for each lok‐bar were measured using a farmer‐type ionization chamber (PTW30013) and the I'mRT phantom (IBA Dosimetry GmbH). Measurement points were between gantry angles of 110 and 180°. The treatment apparatus was a NovalisTx (Brainlab AG); X‐ray energies were set at 6 MV and 10 MV. In the presence of each lok‐bar, the radiation dose was measured in accordance with 10 volumetric modulated arc therapy–stereotactic body radiation therapy (VMAT‐SBRT) plans for lung cancer. Artifacts were seldom observed in the CT scans of the HM‐bar. The attenuation rate of each lok‐bar was higher when the X‐ray energy was set at 6 MV than at 10 MV. The highest attenuation rate in the VL‐bar was observed at a gantry angle of 112°; the rates were 22.4% at 6 MV and 19.3% at 10 MV. Similarly, the highest attenuation rate for the HM‐bar was also observed at a gantry angle of 112°; the rates were 12.2% and 10.1% at 6 MV and 10 MV, respectively. When the VL‐bar was evaluated, the isocenter dose of the VMAT‐SBRT plans was attenuated by 2.6% as a maximum case. In the case of the HM‐bar, the maximum attenuation was 1.4%. In the measurements of each VMAT‐SBRT plan, the difference of the dose attenuation rate between the VL‐bar and HM‐bar was approximately 1%. The HM‐bar could be used to minimize the occurrence of artifacts and provide good images in CT scans regarding radiotherapy planning and dose calculation. It can be used for patient therapy at hospitals to provide accurate dose delivery because of its low X‐ray scattering and absorption characteristics.
Highlights
With the development of radiation therapy devices, highly precise radiation therapies such as stereotactic body radiation therapy[1,2] (SBRT) and volumetric modulated arc therapy (VMAT)[3] are available in many facilities
In the VLbar, the metal artifacts from two pins and the component used to stabilize the couch were obvious
The maximum difference in the values calculated by means of computed tomography (CT) was considered to be approximately 680 HU when using the Varian ExactTM lok-bar (VL-bar) (Fig. 3(e))
Summary
With the development of radiation therapy devices, highly precise radiation therapies such as stereotactic body radiation therapy[1,2] (SBRT) and volumetric modulated arc therapy (VMAT)[3] are available in many facilities To administer these highly precise radiation therapy modalities, a small irradiation field and complex irradiation methods are generally used.[1,2,3,4] in comparison with a conventional therapeutic method, higher precision is required to ensure the accuracy of irradiation positioning, repeatability of patient position and patient fixation precision to control body movement during radiation therapy.[5] In particular, absorption-type fixing tools are often used to secure the patient’s position in image-guided radiation therapy.[5,6,7] The American Association of Physicists in Medicine task group report 1768 recommends the use of a couch and various fixtures constructed from carbon-fiber-reinforced plastic because of high mechanical strength, low specific density, and relative radiotranslucence.[9] the possible phenomenon of attenuation of the radiation dose should be considered. The couch-top was modeled and various fixtures were included in the calculation domain[8] as a countermeasure regarding this concern; it was recommended that the influence of immobilization devices on the absorption and scattering of X-rays concerning the prescribed dose be considered
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