Author: Kristina Bjegovic, Yong Chen, Lucia Rodriguez Gonzalez, Marti Roper, Liangzhong Xiang 👨🔬
Affiliation: University of California, Irvine, University of California Irvine, University of Oklahoma Health Sciences Center, University of Oklahoma Health Science Center 🌍
Purpose: Proton therapy is an increasingly popular cancer treatment due to its precision, enabled by the Bragg Peak (BP), which minimizes side effects. This profile necessitates in-patient localization to ensure accurate tumor targeting despite respiration-induced tumor motion. This work integrates 3D Protoacoustic Imaging (PAI) with B-mode ultrasound for real-time range verification within the target volume. The effectiveness of this PAI-US dual-modality system was demonstrated in an in vivo rabbit model.
Methods: A clinical proton beam line (Mevion) was used to treat a rabbit's liver. Using a prior-CT scan, a treatment plan for a liver pseudo-tumor was prepared in TPS, placing the BP at depths of 2.6cm and 4.2cm into the body. During delivery, a 256-channel matrix array, positioned atop the rabbit chest cavity perpendicular to the proton beam, captured PA signals at 225Hz with a custom DAQ (Photosound) and a 128-element linear ultrasound probe, placed laterally to the liver, acquired B-mode ultrasound images at 15Hz to monitor respiratory motion. All data was processed in MATLAB.
Results: Fifteen ultrasound frames were taken per breath cycle, demonstrating 3mm superior displacement of the liver boundary due to respiration. PA signals gathered in between B-mode ultrasound frames were averaged and reconstructed for both BP distances. Taking into account the geometric relationship between the PA and US transducers, the reconstructed BP was overlaid on US images, localizing the dose deposition within (4.2cm) and outside of (2.6cm) the target volume.
Conclusion: Using the dual-modal PAI-US system, the BP was localized within US images taken during treatment in vivo. The visualized respiratory motion, known to be an order of magnitude larger in humans, demonstrated the change in BP localization during treatment. Translating this monitoring system into the clinic would allow for real-time adaptive radiotherapy, reducing off-target irradiation due to respiratory motion and improving the outcomes of patients.