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Published on: April 13, 2016
Commissioning of a gantry-mounted synchrocyclotron for preclinical FLASH studies utilizing spread-out Bragg peaks
Michael Lowe1, Hailei Zhang1, Daniel Owen2
1Department of Radiation Oncology, WashU Medicine, St. Louis, Missouri, USA.
Background:
Preclinical studies demonstrate the benefits of ultra-high dose-rate (FLASH) radiation, reducing normal tissue toxicity while maintaining tumoricidal effects. Proton FLASH (pFLASH) studies typically use transmission beams, missing the normal tissue-sparing advantage of the spread-out Bragg peak (SOBP).
Purpose:
This study aims to propose and implement a series of modifications to enable a clinical Mevion S250i gantry-mounted synchrocyclotron to deliver pFLASH within the SOBP of the proton beam.
Methods:
A clinical synchrocyclotron was modified to enable FLASH proton beam delivery using the Mevion FLASH accessory kit, a commercially available tool that allows for the delivery of a single spot SOBP at FLASH dose rates. To ensure accurate beam monitoring, a Faraday cup was utilized to measure the integral charge per delivery at different dose rates and calibrate the FLASH transmission ion chamber (FLASHTic), which is integrated in the FLASH accessory mount attached to the nozzle of the gantry. The FLASHTic was specifically designed to prevent saturation at the dose rates associated with FLASH. To generate the desired single spot SOBP, boron carbide absorbers, a range modulating hole filter, and an 11-mm-diameter circle brass aperture were employed on the FLASH nozzle mount.
Results:
The results indicate that the FLASHTic measurements demonstrated a strong correlation with the Faraday cup post calibration, suggesting that the FLASHTic can be effectively utilized for both monitoring and terminating the proton beam. The 80%-80% width of the SOBP was 2.01 cm. The peak dose rate at the SOBP proximal peak reached 105.03 Gy/s, with an average of 96.18 Gy/s over five days. Transitioning between FLASH and clinical mode required less than one hour without affecting the clinical beam.
Conclusions:
The commissioning of a 230 MeV proton synchrocyclotron for SOBP FLASH delivery was achieved, providing a platform for preclinical small animal studies on pFLASH effects.
