Technical Realization of FLASH Proton Irradiation with Synchrocyclotron

Presenting author:
Tianyu Zhao, PhD

By Harold Paganetti, PhD

Radiation dose delivery at ultra-high dose rates is currently being studied at various institutions with pre-clinical experiments. Ultra-high dose rates refers to rates above 40 Gy/second compared to 2 Gy/minute in a typical clinical setting. This so called FLASH mode has been shown to greatly reduce normal tissue damage while maintaining tumor control compared to conventional dose rate delivery. The effect has been demonstrated in photon, electron and proton beams, but pre-clinical data indicate that FLASH effects likely occur at doses higher than currently delivered in standard fractionation regimens. The underlying mechanisms of FLASH are not entirely clear, making FLASH one of the prime research topics in many clinical radiation biology labs. One open question is the impact of the time over which the dose rate is averaged and how variations in dose rate over the course of a treatment fraction play a role.

The abstract FLASH Proton Irradiation using a Synchrocyclotron: Proof of Concept reports on the technical realization of FLASH proton irradiation with a synchrocyclotron aiming at studying instantaneous versus average dose rates in the future. They achieved average dose rates of 216 Gy/s at the Bragg peak with a field size of 1.2 cm full-width at half-maximum (FWHM).

For their system, the authors stated, “The instantaneous dose rate measured in the Bragg peak was 1.67×10⁴Gy/second. We ran 24 beams repeatedly with each beam delivering 1000 Gy (3000 pulses with every pulse carrying 54 pC of protons, pulse repetition rate was 648 Hz). We estimate the standard deviation of the dose rates in beams running over 1 second was 2.2% (ranging from -3.3% to 4.2%) measured in beams running for 4.6 seconds.”

For their experiments, “The majority of the beams delivered with 20 µs pulse width lasted about 80 ms. It delivered about 17Gy per beam, closer to what we would use clinically than longer beam-on time at the highest dose rate.”

Asked about the reproducibility of the dose rate, the authors stated, “We estimate the standard deviation of the dose rates with our machine was 1.8% (ranging from -5.2% to 4.0%). The dose agreement between the ion chamber and the Monte Carlo simulation was 4.7%.”

Even if the mechanisms of FLASH would be fully understood, the clinical translation will be challenging not only due to technical barriers but also challenges in dosimetry. Furthermore, dose rate averaging will likely play a role in multi-field treatments.

FLASH Proton Irradiation using a Synchrocyclotron: Proof of Concept was released onDemand on Monday, October 26 in the Science Center, as part of Scientific session (SS) 23.

Published on: October 27, 2020