Characterization of 250 MeV Protons from the Varian ProBeam PBS System for FLASH Radiation Therapy

Abstract

Shoot-through proton FLASH radiation therapy has been proposed where the highest energy is extracted from a cyclotron to maximize the dose rate (DR). Although our proton pencil beam scanning system can deliver 250 MeV (the highest energy), this energy is not used clinically, and as such, 250 MeV has yet to be characterized during clinical commissioning. We aim to characterize the 250-MeV proton beam from the Varian ProBeam system for FLASH and assess the usability of the clinical monitoring ionization chamber (MIC) for FLASH use. We measured the following data for beam commissioning: integral depth dose curve, spot sigma, and absolute dose. To evaluate the MIC, we measured output as a function of beam current. To characterize a 250 MeV FLASH beam, we measured (1) the central axis DR as a function of current and spot spacing and arrangement, (2) for a fixed spot spacing, the maximum field size that achieves FLASH DR (ie, > 40 Gy/s), and (3) DR reproducibility. All FLASH DR measurements were performed using an 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 integral depth dose were 37.58 and 37.69 cm, and spot sigma at the isocenter were σx = 3.336 and σy = 3.332 mm, respectively. The absolute dose output was measured as 0.343 Gy*mm2/MU for the commissioning conditions. Output was stable for beam currents up to 15 nA and gradually increased to 12-fold for 115 nA. Dose and DR depended on beam current, spot spacing, and arrangement and could be reproduced with 6.4% and 4.2% variations, respectively. Although FLASH was achieved and the largest field size that delivers FLASH DR was determined as 35 × 35 mm2, the current MIC has DR dependence, and users should measure dose and DR independently each time for their FLASH applications.

Commissioning a 250 MeV research beamline for proton FLASH radiotherapy preclinical experiments

“…Figure 5b shows a magnified view of Figure 5a near the 15 Gy region, where the same plan delivered at the same requested nozzle beam current was repeated five times. The variation, calculated as the difference between maximum and minimum, was 0.34 Gy, which corresponds to 2.3%, consistent with the reported values 33,34 . As shown in Figure 5b, the five repetitions with different delivered doses can be well separated by the SICA detector signal, demonstrating the feasibility and suitability of using the SICA detector placed upstream as an in vivo dosimeter to estimate the dose delivered at the isocenter.…”

“…An off‐the‐shelf brass aperture system was also presented to provide the desired dose profiles for small animal experiments. This experience can serve as a valuable reference for other centers interested in implementing FLASH radiotherapy preclinical research, especially those equipped with a similarly designed MIC, which suffers from saturation during FLASH delivery, including most ProBeam centers 13,33 …”

“…This work describes our experience in commissioning a 250 MeV proton research beamline of the Varian ProBeam system (Varian Medical Systems, a Siemens Healthineers company, Palo Alto, California, USA) for use in small animal experiments. In particular, the saturation effect of the monitor ionization chamber (MIC) at high beam currents, from which most ProBeam centers as well as other centers equipped with a similarly designed MIC also suffer during FLASH delivery, 13,33 is systematically investigated and characterized. Also, a novel in vivo dosimetry system is proposed to mitigate the challenges associated with the saturated MIC and ensure an accurate correlation between toxicity results and received dose for animals.…”

Development and characterization of a versatile mini‐beam collimator for pre‐clinical photon beam irradiation

“…In this study, EBT‐XD films were used as dosimeters, also for UHDR irradiations at SCD C . Although there are not many dedicated studies about the response of EBT‐XD films in UHDR for photon beams, a dose overestimation of 3% – 5% for a dose rate of over 40 Gy/s was reported for protons 30–32 . The dose overestimation trends to increase with increasing dose rate, for instance, 12% overestimation was reported for dose rates of 7500 Gy/s 30 .…”

“…Although there are not many dedicated studies about the response of EBT-XD films in UHDR for photon beams, a dose overestimation of 3% -5% for a dose rate of over 40 Gy/s was reported for protons. [30][31][32] The dose overestimation trends to increase with increasing dose rate, for instance, 12% overestimation was reported for dose rates of 7500 Gy/s. 30 Other studies have reported Abbreviations: ctc, center-to-center distance; FWHM, full width at half maximum; PVIR, peak-to-valley intensity ratio.…”