The percent depth dose and dose profiles of the CONV beam are shown in Fig. When both the collimator window and target area were set as 2 × 2 cm, the dose uniformity ratios (ratio of the maximum dose divided by the minimum dose) were 1.43 and 1.25 at depths of 4 and 16 mm, respectively ( Fig. As the depth increased, the dose rates dropped rapidly, but the dose uniformity in the irradiation field was improved. In our experiments, the dose rate of FLASH irradiation in mice was 700–1200 Gy/s. The percentage depth dose (PDD) in the sample was adjusted by changing the beam current (1–10 mA) and energy (6–8 MV) of the superconducting linac. Both measurements and MCC results showed that in an irradiation field with a diameter of 6 cm and at a depth of over 15 cm in water, the HEX dose rate produced by PARTER was higher than 50 Gy/s ( Fig. This discrepancy might have been induced by beam energy attenuation and has been corrected in the total dose. The time histories of the dose rate given by the scintillator monitor and the FCT showed good agreement (<1%), while approximately 5% discrepancy was observed after approximately 1 ms when the beam power was higher than 40 kW ( Fig. 1d, e) and agreed with the preset parameter of the linac. The time structure, mainly include the length of the macro pulse and period of the micro pulse, was measured using the FCT and CeBr 3 scintillator ( Fig. The absolute dose measured using medical radiochromic films (EBT3) showed good agreement (discrepancy < 4%) with MCC results, which were based on the electron number given by the FCT device. The dose rate was a critical parameter that was carefully measured to monitor the implementation of FLASH. HEX: high-energy X-rays PARTER: platform for advanced radiotherapy research PMMA: poly(methyl methacrylate) FCT: fast current transformer MCC: Monte Carlo computing R:radius. (i) 2D dose distribution and profiles in the x-direction in each film. (h) Four EBT3 films were mounted at a depth of 4–16 mm of the PMMA phantom behind the collimator window (2 × 2 cm) and were irradiated for 15 ms by a HEX beam of 6 MeV/5.35 mA. The dose distribution in the phantom was measured before every biological experiment. Smaller radii show higher mean dose rates because of the center-edge dose attenuation, and the maximum mean dose rate within R10 mm, achieved at 0.5 cm depth, was approximately 750 Gy/s. The mean dose rates within three different radii (R10, R20, and R30 mm) in the EBT3 films agree with the MCC results based on the beam current given by FCT with a discrepancy of 1% at most of the points and approximately 3% near the surface. (g) The mean dose rate was given by the total dose measured by film divided by the time of delivery measured by the FCT and Scintillator. To measure the dose rate and distribution, six EBT3 films were mounted at various depths from 4 mm to 55 mm in the solid water phantom, which is located at the sample position 7 cm behind the rear surface of the target chamber and irradiated using a HEX beam of 7 MeV/3.8 mA. (d) The current and pulse width of the linac measured using FCTs with a time resolution of nanoseconds, installed on the beamline, and (e, f) the time histories were compared with those obtained using the scintillator. (b) A mouse was fixed on the PMMA holder while being exposed to FLASH irradiation, and (c) a EBT3 film was stuck on the backside of the holder to verify the location of the irradiation field in the mouse and to assist with dose measurement. (a) The schematic diagram of the HEX-FLASH experiment on PARTER. 1 Parameters and results of the basic HEX-FLASH experiment on PARTER.
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