SU-F-BRE-03: Consideration of a Track-Interaction Model for Radiochromic Film Response

Authors


Abstract

Purpose:

Conventional methods for characterizing the energy response of radiochromic film (RCF) typically involve assessing changes in response when exposed to various beam qualities and use Monte Carlo to determine absorbed dose. These methods represent RCF as a dose integrator of a homogeneous energy deposition volume. Apparent film saturation, nonlinearity, and intrinsic energy dependence are unpredicted with conventional methods. Recent work has shown significant RCF intrinsic energy dependence, which limits its use in absolute dosimetry. This work introduces a track-interaction model (TIM) for RCF and assesses its ability to predict total energy response.

Methods:

A TIM based on Katz single-hit theory was developed to accumulate energy flux along particle tracks within active crystals, represented as (1×1×20)um3 prisms about the Gafchromic™ EBT3 active volume using MCNP5 and Matlab. Energy flux contributed to film response only if near the threshold energy for polymerization in polydiacetylenes (2.5eV/monomer). Energy deposition in excess of maximum efficiency represented crystal saturation and did not contribute to film response. The TIM was applied to RCF exposed in air to various monoenergetic photon beams and Co-60. Geometric distribution of energy flux was found for each beam quality in a (1×1)mm2 RCF area. RCF response relative to Co-60 absorbed dose-to-water (S_TIM) was determined and compared to published values (S_PUB).

Results:

TIM successfully predicted that lower energy radiation is less effective at inducing polymerization, though the magnitude of the phenomenon was overpredicted. S_TIM was −29% and +20% for 20 and 40 keV, respectively. This agreed qualitatively with S_PUB of −27% and +16%. TIM-generated sensitometric curves contained the non-linearity and saturation apparent in RCF.

Conclusion:

This work indicates the possibility for TIMs to predict changes in RCF response to various energies. Future work will refine TIM by considering size distributions of active elements, other RCF formulations, and response in complex treatment fields.

Acknowledgements: The authors wish to thank Dr. Christopher Soares at NIST (Gaithersburg, MD) for performing film measurements and Dr. David Lewis of Ashland, Inc. (Wayne, NJ) for making the EBT3 film available to us.

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