Feasibility study of using fast low-dose pencil beam proton and helium radiographs for intrafractional motion management

Purpose
This study aims to evaluate the feasibility of using fast, low-dose proton (pRad) and helium (HeRad) radiography for intrafractional motion management. This approach uses pencil ion beam delivery systems, modern particle imaging detectors and fast image reconstruction.

Methods
A plastic respiratory phantom underwent four-dimensional computed tomography ($_4$DCT) using a commercial X-ray scanner, experimental pRad with a continuous proton beam from a clinical serial cyclotron, and experimental pRad and HeRad with pulsed proton and helium beams from a synchrotron-based ion therapy facility. Open-source patient $_4$DCT data were used in a Monte Carlo simulation study to evaluate pRad and HeRad in a realistic patient geometry. Treatment plans involving mixed carbon-helium beams were calculated using matRad and simulated in TOPAS.

Results
The experimental pRad achieved a temporal resolution of 8 fps for the cyclotron-based facility, while both pRad and HeRad achieved 2 fps for the synchrotron-based facility within a 10 cm × 10 cm region of interest. pRad reconstructed the respiratory phantom motion pattern with a dose of less than 2 µGy per image. ... mehrIn simulations of mixed carbon-helium beams, HeRad, both integral and single iso-energy, detected water equivalent thickness differences with sub-millimeter accuracy across different phases of the patient’s $_4$DCT data.

Conclusion
This study demonstrates that low-dose small-field proton and helium radiography, utilizing pencil beam scanning, can effectively monitor intrafractional anatomical displacements with millimeter-level spatial accuracy and sub-second temporal resolution. Current particle imaging and beam delivery technologies have the potential to enable real-time patient monitoring in promising mixed ion beam therapy.