Room: Stars at Night Ballroom 2-3
Purpose: The finite range of proton therapy, while allowing for rapid dose fall-off beyond the target, hinders exit dose-based image guidance. Here we present our first results of a prototype proton portal imaging (PPI) system that images within the spread-out Bragg peak (SOBP) region in proton therapy (PT).
Methods: We fabricated a proof-of-concept PPI system to image a phantom placed in a 179MeV therapy proton beam (Range=15.1cm, Modulation=10.4cm), whereby neutrons generated by proton interactions within the phantom can be utilized for imaging. The phantom was thicker than the maximum range of the proton beam, and consisted of low-density discs and an air cavity sandwiched between solid water. The prototype detector system used for neutron imaging is a bootstrapped NeutronOptics imager based on a CCD camera that acquires transmission images generated by exit neutrons incident on a 200x200mmÂ² scintillator (0.2mm thickness of â?¶LiF-ZnS.) A lens-mirror system focuses the image onto a 2048x2048 pixels CCD chip. â?¶LiF-ZnS was selected due to availability, and neutron converter property of â?¶Li.
Results: PPI acquired transmission neutron radiography images of a low contrast discs and an air cavity sandwiched in solid water. The neutrons and X-rays produced by the spallation reaction of the proton beam are forward scattered and interact with the scintillation screen to create transmission imaging. PPI imaging, with estimated resolution of ~3mm and produced at doses of 140cGy to the phantom, was comparable but slightly lower imager quality, to portal imaging from a conventional 6MV linac.
Conclusion: Portal imaging was demonstrated using a contrast phantom irradiated with a therapy proton beam. Low density inserts within the SOBP were visualized. Lower dose imaging should be possible with thicker scintillators. We believe that with optimization PPI can be used for treatment verification, and is potentially useful for in vivo dosimetry in PT.