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Background: Ultra high dose rate (UHDR) radiotherapy using ridge filter is a new treatment modality known as conformal FLASH that, when optimized for dose, dose rate (DR), and linear energy transfer (LET), has the potential to reduce damage to healthy tissue without sacrificing tumor killing efficacy via the FLASH effect. Purpose: Clinical implementation of conformal FLASH proton therapy has been limited by quality assurance (QA) challenges, which include direct measurement of UHDR and LET. Voxel DR distributions and LET spectra at planning target margins are paramount to the DR/LET‐related sparing of organs at risk. We hereby present a methodology to achieve experimental validation of these parameters. Methods: Dose, DR, and LET were measured for a conformal FLASH treatment plan involving a 250‐MeV proton beam and a 3D‐printed ridge filter designed to uniformly irradiate a spherical target. We measured dose and DR simultaneously using a 4D multi‐layer strip ionization chamber (MLSIC) under UHDR conditions. Additionally, we developed an "under‐sample and recover (USRe)" technique for a high‐resolution pixelated semiconductor detector, Timepix3, to avoid event pile‐up and to correct measured LET at high‐proton‐flux locations without undesirable beam modifications. Confirmation of these measurements was done using a MatriXX PT detector and by Monte Carlo (MC) simulations. Results: MC conformal FLASH computed doses had gamma passing rates of >95% (3 mm/3% criteria) when compared to MatriXX PT and MLSIC data. At the lateral margin, DR showed average agreement values within 0.3% of simulation at 100 Gy/s and fluctuations ∼10% at 15 Gy/s. LET spectra in the proximal, lateral, and distal margins had Bhattacharyya distances of <1.3%. Conclusion: Our measurements with the MLSIC and Timepix3 detectors shown that the DR distributions for UHDR scenarios and LET spectra using USRe are in agreement with simulations. These results demonstrate that the methodology presented here can be used effectively for the experimental validation and QA of FLASH treatment plans. [ABSTRACT FROM AUTHOR]

Purpose: Uncertainty in proton range can be reduced by proton computed tomography (CT). A novel design of proton CT using a multiple‐layer ionization chamber with two strip ionization chambers on the surface is proposed to simplify the imaging acquisition and reconstruction. Methods: Two strip ionization chambers facing the proton source were coupled into a multiple‐layer ionization chamber (MLIC). The strip chambers measured locations and lateral profiles of incident proton beamlets after exiting the imaging object, while the integral of depth dose measured in the MLIC was translated into the residual energy of the beamlet. The simulation was performed at five levels of imaging dose to demonstrate the feasibility and performance expectations of our design. The energy of the proton beamlet was set to 150 ± 0.6 MeV. A collimator with a round slit of 1 cm in diameter was placed in the central beam axis upstream from steering magnets. Proton stopping power ratio (SPR) was reconstructed through inverse radon transform on sinograms generated with proton beamlets scanning through an imaging phantom from a half‐circle gantry rotation. The imaging phantom was 10 cm in diameter. The base was made of water‐equivalent material holding 13‐tissue equivalent inserts constructed according to ICRP 1975 (Task Group on Reference Man. "Report of the Task Group on Reference Man: A Report", Pergamon Press 23, 1975). All inserts were 1 cm in diameter with materials ranging from lung to cortical bone. Percentage discrepancies were reported by comparing to the ground truths. The imaging dose and quality were also evaluated. Results: The maximum deviation in reconstructed proton SPR from the ground truths was reported to be 1.02% in one of the 13 inserts when the number of protons per beamlet passing through the slit dropped to 103. Imaging dose was correlated linearly to incident protons and was determined to be 0.54 cGy if 5 × 102 protons per beamlet were used. Imaging quality was acceptable for planning purpose and held consistently through all levels of imaging dose. Spatial resolution was measured as five line pairs per cm consistently in all simulations varying in imaging dose. Conclusions: Proton CT using a multiple‐layer ionization chamber with two strip ionization chambers on the surface simplifies data acquisition while achieving excellent accuracy in proton SPR and acceptable spatial resolution. The imaging dose is lower compared to kV CBCT, making it potentially a great tool for localization and plan adaption in proton therapy. [ABSTRACT FROM AUTHOR]