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A Quenching Correction Method for Volumetric Scintillation Dosimetry of Proton Beams

D Robertson

D Robertson1,2*, D Mirkovic1, S Beddar1, (1) UT MD Anderson Cancer Center, Houston, TX, (2) University of Texas Graduate School of Biomedical Sciences, Houston, TX

TU-A-BRB-5 Tuesday 8:00:00 AM - 9:55:00 AM Room: Ballroom B

Purpose: Liquid scintillator dosimetry has the potential to provide fast, high-resolution, three-dimensional dosimetry of radiation beams. However, organic liquid scintillators exhibit a non-linear response at the high linear energy transfer (LET) values characteristic of ion beam Bragg peaks. The purpose of this study was to develop a method to correct for this quenching effect in scintillators used for volumetric proton beam dosimetry.
Methods: Scintillation light from a miniature (4 mm^3) liquid scintillator detector was measured along the central axis of a 161.6 MeV therapeutic proton pencil beam. Three-dimensional dose and LET distributions were calculated for 85.6, 100.9, 144.9, and 161.6 MeV beams using a validated Monte Carlo model. LET values were also calculated using an analytical formula. The empirical parameters of a quenching correction model were determined by least squares fitting of the model to the signal from the miniature detector and the Monte Carlo data for 161.6 MeV protons. The scintillation light distribution in a tank of liquid scintillator was measured with a CCD camera at all four energies. The quenching model and LET data were then used to correct the light distribution measured in the scintillator tank. The quality of agreement between the calculated dose and corrected light signal was assessed by the percent difference on the central axis and by gamma analysis.
Results: The calculated and measured light signals agreed within +/-3% on the central axis of the proton pencil beam for all energies except 85.6 MeV, where the agreement was +/-4%. The gamma analysis passing rate exceeded 95% with gamma criteria of 2%, 2 mm.
Conclusion: We have developed a method to correct for scintillator quenching in a large detection volume with sufficient accuracy for dosimetric purposes. This method requires prior knowledge of the LET distribution of the beam and the quenching factors for the scintillator.

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