![]() ![]() The East Japan Heavy Ion Center (EJHIC), located in Yamagata University's Faculty of Medicine, is the 7 th such facility in Japan and started treatment with carbon ions in February 2021. However, it has recently been reported that the mouthpiece has an error of up to 13% with the HU‐SPR calibration curve, depending on the material. Furthermore, because the physical thickness of the mouthpiece is not large, it is believed that the error is included in the error on the HU‐SPR calibration curve or the expected beam range error considered in the margin of the target volume. In a CT image, however, it is difficult to accurately distinguish the teeth from the mouthpiece, and it is impractical and time‐consuming to accurately contour the mouthpiece on CT images slice by slice to replace its value with an accurate SPR value. Thus, it is necessary to replace the SPR value derived from the HU‐SPR curve with an accurate value.įor head and neck (HN) treatment in radiation therapy, the mouthpiece has been used to reduce setup errors, such as by enhancing positioning accuracy, improving inter‐fractional reproducibility, and alleviating the dose of the tongue. In the case of non‐human materials, the SPR converted through the HU‐SPR calibration curve differs significantly from the expected value because the CT number is affected by beam hardening and the SPR depends on electron density, whereas the CT number depends on both electron density and atomic number.įor example, graphite has a large electron density however, because its small atomic number leads to a small CT number, its SPR value deviates considerably from the HU‐SPR curve. Nevertheless, it is known that the HU‐SPR calibration curve has an error of approximately 3% or more for the material of the human body. ![]() The ion energy required to locate the Bragg peak at the tumor position is then determined using the integral of the SPR information along the beam path. In particle therapy, the computed tomography (CT) number of the human body obtained by a CT scanner is converted into a stopping power ratio (SPR) value relative to water using the Hounsfield unit (HU)‐SPR calibration curve. For a beam passing through the mouthpiece in HN treatment, it would be practical to consider a mouthpiece margin of 1 mm to avoid beam range errors if ions pass through the mouthpiece. From this error, it was evaluated that for a mouthpiece with a thickness of 10 mm, a beam range error of approximately 0.4 mm can occur, and for a mouthpiece with a thickness of 30 mm, a beam range error of approximately 1 mm can occur. The WEL value of the mouthpiece sample had an error of approximately 3.5% in the HU‐SPR calibration curve. Compared with the Hounsfield unit (HU)‐SPR calibration curve used at the EJHIC, the SPR error on each measured and theoretical value was calculated. To calculate the difference between the theoretical and measured values, the theoretical CT number and SPR value of the sample were calculated using the stoichiometric calibration method. The average value of the water equivalent length (WEL) of each sample was obtained from the difference between the range of the Bragg curve and the thickness of the sample. The integral depth dose of the Bragg curve with and without these samples was measured for 292.1, 180.9, and 118.8 MeV/u of the carbon‐ion pencil beam using an ionization chamber with concentric electrodes at the horizontal port of the EJHIC. Erkoflex and Erkoloc‐pro from ERKODENT and samples that combined Erkoflex and Erkoloc‐pro were computed tomography (CT)‐scanned using head and neck (HN) protocol at the East Japan Heavy Ion Center (EJHIC), and the values were averaged to obtain the CT number. ![]() ![]() The errors on the stopping power ratio (SPR) of mouthpiece samples from ERKODENT were evaluated. ![]()
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