(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: Technologies are coming up with 3D online verification methods using detectors placed at the tray level of the Clinac gantry. Dose-per-pulse for FFF beams are expected to increase as we go near to the target. The purpose of this work is to measure ion-recombination effect of various ion-chambers at tray-level measurement setup for high-dose-per-pulse 6MV FFF beam, using two-voltage method and study the chamber characteristic using Jaffe-plot.
Methods: Measurements were performed on Varian Clinac-iX Linear accelerator upgraded to deliver 6MV FFF beam with a maximum nominal dose-rate of 1400 MU/min. Different volume ionization chambers FC65G-0.65cc, CC13-0.13cc, Exradin A14, A11 Parallel-plate and PPC05-0.015cc were used with DOSE1 electrometer. The electrode separation of the chambers were 3.1mm, 3.0mm. 2mm, 2mm and 0.6mm respectively. For 10x10sq.cm field-size charges were measured for voltages from 50V to 300V insteps of 50V with 2cm buildup. After 5 minutes of pre-irradiation, three consecutive readings corrected for chamber leakage, temperature and pressure variations were taken. Ion recombination Pion was calculated using AAPM TG51 two-voltage method for 150V & 300V. Inverse of voltage and normalized inverse of charge were plotted.
Results: The Jaffe-plots for all chambers showed linear relationship between inverse of voltage and normalized inverse of charge at tray-level. The ion recombination measured for FC65G, CC13, A14, A11pp and PPC05 chambers were 1.0251, 1.0247, 1.0047 1.0113 and 1.0046 respectively. The Pion for FC65G and CC13 were significantly similar and also higher despite the volume difference. The ion recombination effect was higher for chambers with electrode separation more than 2mm.
Conclusion: Ion chambers with less electrode separation exhibited less ion recombination effect for 6MV FFF beams. Thus the result recommends the ion-chambers with a electrode separation less than 2mm for 6MV FFF beams at Clinac tray-level measurement.
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