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Low-Energy Spectral Changes in NIPAM Polymer Gel Dosimeters

L Turner*, B Smith, W Culberson, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI


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

Room: AAPM ePoster Library

Purpose: NIPAM polymer gel dosimeters (PGDs) are 3D dosimeters that are used to measure 3D dose distributions from clinical and preclinical radiation beams. This work aims to model the inherent low-energy spectral changes in NIPAM PGDs when irradiated using a 220 kVp x-ray beam and the absorbed dose to the medium.
Methods: Monte Carlo N-particle (MCNP) simulations were performed using a 220 kVp x-ray spectrum generated using SpekCalc and a 2-cm cube of either NIPAM PGD or water. Photon spectra were tabulated using an F1 current tally with 0.5 keV energy bins at the top surface of the cube, in the center of the cube (1 cm depth), and at the bottom surface of the cube (2 cm depth). Dose was calculated using a *F8 pulse height tally in a 0.2-cm3 cube of water on the central axis at a depth of 1.5 cm in both the NIPAM and water cubes.
Results: Spectral changes from the nominal 220 kVp x-ray beam were observed at several locations in both water and NIPAM PGD cells. The most significant change was an increase in the mean photon energy by ~2 keV due to beam hardening. Another prominent change was the presence of a peak located at ~50 keV. The absorbed dose in the 0.2-cm3 water cells differed by 0.84% percent.
Conclusion: Low-energy spectral changes in NIPAM dosimeters include beam hardening and a peak at ~50 keV. Monte Carlo simulations show these spectral changes affect the absorbed dose minimally. A Monte Carlo model has been created to investigate absorbed dose differences in gel dosimeters. Simulated energy changes are not expected to cause an observable energy dependence throughout the dosimeter. Future work will include benchmarking the calculations with a dosimeter with a well-known energy dependence.


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