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Determination of the Recombination Correction Factor for a Parallel-Plate Chamber Irradiated by Synchrotron-Based FLASH Proton Beams

M Yang1*, X Wang1, M Chen2, U Titt1, S Tootake3, T Katayose3, Y Li1, X Zhang1, F Poenisch1, M Umezawa3, S Frank1, A Koong1, X Zhu1, (1) UT MD Anderson Cancer Center, Houston, TX, (2) Ruijin Hospital, Huangpu, Shanghai, CN, (3) Hitachi, Tokyo, Japan.

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Recently, the delivery of proton beams with ultra-high dose rate was realized on our clinical synchrotron-based proton system, with the theoretically estimated dose rate ranging from 600Gy/s to 5400Gy/s. The goal of this study was to determine the recombination correction factor (RCF) for the advanced Markus chamber when irradiated with such synchrotron-based ‘FLASH’ proton beams, which has not been reported before.


Methods: representative beam conditions were selected in this study, which covered the full range of dose and dose rates available on our system. To further increase the spread of dose rates, the measurement was done at two different depths: the surface and the Bragg peak (~5.3 cm). Four voltages were used for each measurement condition: 100V, 200V, 300V and 400V. The RCFs were calculated based on the model for the continuous beam and pulsed beam, respectively. For each model, both extrapolation technique and two-voltage technique (300V–100V) were used to determine RCF. In addition, an empirical rational function was used to directly fit the ratio of the collected charge to the voltage.


Results: RCFs determined based on the continuous beam model (both extrapolation and two-voltage technique) and the rational function agreed well with each other: the differences less than 0.011. The RCFs calculated based on the pulsed beam model had either extremely large values (up to 1.16) or values less than 1, which did not agree with the fact that small increase of chamber reading (< 1%) was observed when the voltage increased from 300V to 400V. Based on the continuous beam model, the RCF increased from 1.004 to 1.034 as the measured dose rate increased from 367 Gy/s to 7134 Gy/s.


Conclusion: continuous beam model should be used to determine RCF for synchrotron-based proton beams. Additional dosimetry measurements with other dosimeters are needed to confirm our calculated RCFs.

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Keywords

Absolute Dosimetry, Ion Recombination

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – experimental dosimetry

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