Purpose: The National Nuclear Security Administration’s Office of Radiological Security (NNSA/ORS) implements a radiological risk reduction program which seeks to minimize or eliminate the use of high activity radiological sources, including Cs-137, by replacing them with non-radioisotopic technologies, such as x-ray irradiators. The goal of this study funded by ORS is to determine absorbed depth dose equivalence between gamma and x-ray technologies based on a Monte Carlo-driven approach, to remove any confounding factors in radiation experiments using different sources.
Methods: Using Monte Carlo simulations, we calculate the 3-D distribution of the absorbed dose in a realistic digital mouse phantom (MOBY) irradiated with Cs-137 in a Mark I-68A and with X rays at 160kVp, 225kVp and 320kVp in a XRAD320 Unit. Precise geometries were each modeled in the Geant4 application for tomographic imaging (GATE), including shape, materials, and photon energy spectra. Simulations were calibrated using in site measurements with DOSE-MAPTM film cassettes. Sub-millimeter dose distributions for MOBY are computed at the voxel and organ level.
Results: our Monte Carlo model shows that the absorbed dose becomes more uniform as the energy of the photons increases, due to the different photon penetration. Higher doses are deposited in bone, while it is lower and very consistent in other tissue. Our Monte Carlo model provides equivalence tables for the absorbed doses in each device at organ and voxel level.
Conclusion: this work provides a detailed Monte Carlo study intended to compare the dose absorbed in x-ray irradiation systems with the Cs-137 irradiation standard. We show that doses are uniform and consistent with our measurement in non-bone tissue, while the dose in bone can be a factor of 2 higher for 160kVp. This study might help replacing Cs-137 gamma irradiators in small-animal research studies to reduce security risks associated with cesium chloride proliferation.
Funding Support, Disclosures, and Conflict of Interest: Funding support: This work was funded by Sandia National Laboratories. Disclosures and Conflict of Interest: None.
Dosimetry, Radiochromic Film, Radiobiology