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Towards Monte Carlo Model Validation of the Optical Properties in Light Guides Used for Scintillation Dosimetry

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E Simiele

E. A. Simiele*, W. C. Culberson , L. A. DeWerd , Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI


SU-I-GPD-T-359 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Develop and validate Monte Carlo models of the optical properties of light guides used in scintillation dosimetry.

Methods: Percent depth dose (PDD) and profile measurements were made in a 6 MV photon beam for a 30×30 cm² field and compared to GEANT4 simulations using a previously validated accelerator model. Simulations were performed with seven physics lists, and the root mean square differences between the simulated and measured profiles were calculated. Next, PDD and percent depth Cerenkov emission (PDCE) profiles were simulated using various optical input parameters and field sizes. Using the optimal optical input parameters, a PMMA light guide was modeled and the relative response vs. depth was simulated for field sizes of 5×5 cm² and 30×30 cm². Measurements were made with a custom-built PMMA light guide detector, and compared to simulations.

Results: Of the physics lists evaluated, EmStandard_option3 was the best compromise between simulation time and agreement with the measured data, and was used for the remainder of this work. The PDCE did not change in magnitude or shape as a function of Cerenkov process input parameters. The PDCE profile increased with refractive index, but the profile shape remained constant. The simulated and measured PDCE curves initially showed disagreements of up to 12%, with the disagreement increasing with depth. By scoring Cerenkov photons that were trapped in the fiber rather than every photon produced in the fiber, the agreement between the simulated and measured data increased, particularly for the 5×5 field where the maximum disagreement was reduced to 1.6%.

Conclusion: Agreement between simulated and measured fiber relative response was achieved for the 5×5 field by scoring Cerenkov photons trapped within the fiber. Work is ongoing to resolve the remaining discrepancies and implement the necessary optical properties to achieve agreement between simulated and measured light spectra exiting the fiber.

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