Purpose: Proton radiography could provide instantaneous imaging for proton therapy, reducing position error and facilitating a more accurate dose calculation. Using a magnetic lens system, it has proven efficacious at high proton energies, but yields mixed results at lower energies. Here, a proton radiography system based on that at the Los Alamos Neutron Science Center (LANSCE) is modeled using the TOol for PArticle Simulation (TOPAS). Clinical to high-energy protons irradiated head, rat, and mouse phantoms in simulation to determine the energy at which proton radiography becomes feasible.
Methods: A proton flux of 108 particles traverses a magnetic lens system forming a Fourier plane (collimation point) and imaging plane downstream. Phantoms are simulated as stacked cylinders of tissue-equivalent material with varying diameters atop a ‘plus sign’ resolution object of bone-equivalent material; parameters changed per phantom to replicate approximate human, rat, and mouse anatomical dimensions.
Results: Percent transmission was assessed through each bone and tissue combination; approximately 40% transmission was the threshold for visibility. While the bone was visible for all energies at the smallest tissue depth, it only became visible at the greatest depth starting at 530 MeV. Resolution across the bone edge was assessed at all energies and depths for the head phantom. Bone is visible with all energies at 4 cm thickness; at 8 cm, a sub-mm resolution image is obtained starting at 330 MeV, with resolution decreasing at higher energies due to edge effects; for 12 cm, at least 530 MeV is required for sub-mm spatial resolution; and 630 MeV required for 16 cm of tissue.
Conclusion: Clinical 230 MeV protons are sufficient only for radiography of shallow tissues or preclinical applications. At least 630 MeV protons would be required to radiograph bone under 16 cm of tissue and image the full depth of an adult patient’s head.
Funding Support, Disclosures, and Conflict of Interest: This project was also supported in part by the Graduate Fellowship in STEM Diversity as part of the GFSD Fellowship.