Three-dimensional proton FLASH dose rate measurement at high spatiotemporal resolution using a novel multi-layer strip ionization chamber (MLSIC) device.

BACKGROUND: Currently, proton therapy is the main radiation treatment modality that can treat deeply seated targets at ultra-high dose rates. The safe translation of FLASH RT into clinic requires dedicated dosimeters capable of measurements at sufficiently high spatiotemporal resolution. PURPOSE: The objective of this work is to demonstrate the feasibility of three-dimensional (3D) measurements of dose rate and dose for FLASH pencil beam scanning (PBS) proton therapy. METHODS: A multi-layer strip ionization chamber (MLSIC) device, along with a reconstruction algorithm, was designed and developed to reconstruct dose and dose rate distribution over a 3D volume. Our MLSIC is composed of 66 layers of strip ionization chamber arrays with total water-equivalent thickness (WET) of 19.2 cm along the beam direction. The first two layers, composed of 128 channels with orthogonal direction with respect to each other, provide the (x,y) coordinate. The other 64 layers contain 32 channels with 8 mm lateral spacing. Data readout at a high-speed of 6250 fps allows spot-by-spot measurement. To prove the concept, PBS proton therapy plans were delivered at conventional and FLASH dose rates. Dose and dose rate information were reconstructed in 3D using an in-house reconstruction algorithm. RESULTS: Ion recombination remained under 1% in the majority of cases. 3D dose reconstruction showed agreement with the treatment planning software; the 3D gamma analysis of the reconstructed dose showed 96.2% (5 mm/5%) and 86.8% (3 mm/3%) passing rates with 10% threshold for the conventional dose rate plan, 99.1% (5 mm/5%) and 92.6% (3 mm/3%) passing rates for the FLASH dose rate plan. 3D dose rate distributions were successfully generated using different definitions. CONCLUSIONS: Our MLSIC device allows obtaining 3D dose and dose rate distribution of PBS proton beams at FLASH dose rates with high spatiotemporal resolution.