Characterization of 250 MeV protons from Varian ProBeam pencil beam scanning system for FLASH radiation therapy

Recently, shoot-through proton FLASH has been proposed where the highest energy is extracted from the cyclotron to maximize the dose rate (DR). Even though our proton pencil beam scanning system can deliver 250 MeV (the highest energy), it is not typical to use 250 MeV protons for routine clinical treatments and as such 250 MeV may not have been characterized in the commissioning. In this study, we aim to characterize 250 MeV protons from Varian ProBeam system for FLASH RT as well as assess the ability of clinical monitoring ionization chamber (MIC) for FLASH-readiness. We measured data needed for beam commissioning: integral depth dose (IDD) curve, spot sigma, and absolute dose calibration. To evaluate MIC, we measured output as a function of beam current. To characterize a 250 MeV FLASH beam, we measured: (1) central axis DR as a function of current and spot spacing and arrangement, (2) for a fixed spot spacing, the maximum field size that still achieves FLASH DR (i.e., > 40 Gy/s), (3) DR reproducibility. All FLASH DR measurements were performed using ion chamber for the absolute dose and irradiation times were obtained from log files. We verified dose measurements using EBT-XD films and irradiation times using a fast, pixelated spectral detector. R90 and R80 from IDD were 37.58 and 37.69 cm, and spot sigma at isocenter were {\sigma}x=3.336 and {\sigma}y=3.332 mm, respectively. The absolute dose output was measured as 0.377 GyE*mm2/MU for the commissioning conditions. Output was stable for beam currents up to 15 nA, and it gradually increased to 12-fold for 115 nA. DR depended on beam current, spot spacing and arrangement and could be reproduced within 4.2% variations. Even though FLASH was achieved and the largest field size that delivers FLASH DR was determined as 35x35 mm2, current MIC has DR dependence and users should measure DR each time for their FLASH applications.Comment: 11 pages, 6 figure

Measurement of the time structure of FLASH beams using prompt gamma rays and secondary neutrons as surrogates

We aim to investigate the feasibility of online monitoring of irradiation time (IRT) and scan time for FLASH radiotherapy using a pixelated semiconductor detector. Measurements of the time structure of FLASH irradiations were performed using fast, pixelated spectral detectors, AdvaPIX-TPX3 and Minipix-TPX3. The latter has a fraction of its sensor coated with a neutron sensitive material. With little or no dead time and an ability to resolve events that are closely spaced in time (tens of ns), both detectors can accurately determine IRTs as long as pile-ups are avoided. To avoid pile-ups, we placed the detectors beyond the Bragg peak or at a large scattering angle. We acquired prompt gamma rays and secondary neutrons and calculated IRTs based on timestamps of the first (beam-on) and the last (beam-off) charged species. We also measured scan times in x, y, and diagonal directions. We performed these measurements for a single spot, a small animal field, a patient field, and a ridge filter optimized field to demonstrate in vivo online monitoring of IRT. All measurements were compared to vendor log files. Differences between measurements and log files for a single spot, a small animal field, and a patient field were within 1%, 0.3% and 1%, respectively. In vivo monitoring of IRTs was accurate within 0.1% for AdvaPIX-TPX3 and within 6.1% for Minipix-TPX3. The scan times in x, y, and diagonal directions were 4.0, 3.4, and 4.0 ms, respectively. Overall, the AdvaPIX-TPX3 can measure FLASH IRTs within 1% accuracy, indicating that prompt gamma rays are a good surrogate for primary protons. The Minipix-TPX3 showed a higher discrepancy, suggesting a need for further investigation. The scan times (3.4 \pm 0.05 ms) in the 60-mm distance of y-direction were less than (4.0 \pm 0.06 ms) in the 24-mm distance of x-direction, confirming the much faster scanning speed of the Y magnets than that of X.Comment: 11 pages, 5 figure

Development of 2-3 mm proton minibeams as a new form of GRID radiotherapy

In this study, we investigated, computationally and experimentally, the feasibility of a new form of proton GRID therapy based on an array of proton minibeams developed at the existing proton therapy clinics. The diameter of the proton minibeams is in the range of 2-3 mm. The purpose of this new form of proton GRID therapy is to further enhance normal tissue sparing during the treatment. The optimal design of the proton minibeam array is based on the figures-of-merit parameters including the peak-to-valley dose ratio (PVDR), dose rate at the Bragg peak and the unwanted neutron dose. Using Monte Carlo code TOPAS we simulated proton pencil-beams that mimic those available at cyclotron-based facilities. We achieved parallel beams of 2-3 mm diameter using physical collimator made of dense materials. The beams are produced via the open holes of the collimator. The spatial pattern of the beam array follows the hexagonal arrangement. We optimized the proton minibeam design by considering different combinations of parameters like beam size, collimator material, center-to-center distance, phantom to collimator distance, and collimator thickness. Verification measurements of the PVDRs using radiochromic films and neutron dose using WENDI-II neutron detector were conducted at two proton therapy facilities: the University of Florida Proton Therapy Institute and the Emory Proton Therapy Center. Results show that using the existing proton pencil beam scanning technique, the optimized proton minibeams can achieve high PVDR values at the entrance of the water phantom and at the same time maintain clinically acceptable dose rates at the tumor depth. Although the neutron dose increases by 20-30 folds (on average) with the use of a collimator, it is still less than the neutron dose produced with double scattered proton beam. Accordingly, the results suggest that it is feasible to develop an array of proton minibeams to further enhance normal tissue sparing during the treatment.Ph.D