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Dose Response of EBT2 Film Modeled as a Bimolecular Reaction

L Perles

L Perles*, D Mirkovic, A Anand, U Titt, R Mohan, UT MD Anderson Cancer Center, Houston, TX

TH-E-BRB-4 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose: This study investigated if the dose-response of EBT2 film can be modeled as a bimolecular reaction of the monomers that composes the active layer of the EBT2 film. The LET dependence of EBT2 films was explored using the models developed in this study.

Methods: To build a dose-response curve a set of films was exposed to pristine proton beams of 161.61 MeV, with doses ranging from 0.93 Gy to 14.82 Gy at a depth of 2 cm in water. The same procedure was applied to a film using a lower energy beam, 85.55 MeV. Because the chemical models predict different values for the maximum optical density of the film, another set of films was exposed to a higher dose, about 200 Gy, to determine which chemical model would better predict the film parameters. Fluence-averaged LET curves were computed by calculating the ratio of the dose and fluence in water. Proton energy spectra were also computed at selected depths for both energies.

Results: The unimolecular and the bimolecular models were able to accurately fit the experimental data, with both having R^2 = 0.9996. The maximum optical density values found were 0.901 +/- 0.050 by the unimolecular model and 1.276 +/- 0.077 by the bimolecular model. Exposing a set of EBT2 films to 200 Gy yielded a measured optical density of 1.360 +/- 0.070, which indicated larger systematic uncertainties in the unimolecular model compared to the bimolecular model.

Conclusions: Although both the unimolecular and the bimolecular models fit the experimental data with similar accuracy, only the bimolecular model could predict the maximum optical density of the EBT2 film with acceptable accuracy. We also observed that the energy spectra at the measurement depths play a role in the LET response of the EBT2 films when described as a bimolecular reaction.

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