Author: Christopher Bartee, Justin Riley DeFrancisco, Marcos Feijoo, Siyong Kim, Tianjun Ma 👨🔬
Affiliation: Blue Physics LLC, Virginia Commonwealth University 🌍
Purpose: Irradiation with FLASH has been shown to decrease normal tissue toxicity while maintaining similar tumor control. To advance FLASH into the clinic, more preclinical investigations are first necessary. Irradiation devices with accurate delivery are rare and expensive. Additionally, existing dosimetry techniques are incompatible with FLASH. We aimed to cost-effectively establish a FLASH infrastructure for preclinical studies by modifying an existing LINAC and developing a novel control system.
Methods: A Varian 21EX LINAC was modified to enable electron FLASH by utilizing photon mode, shutting off the air drive, retracting the target, shifting the carousel to an open port, and tuning waveforms. For beam on/off control, a scintillation detection system was placed in the treatment field to detect electron pulses from the LINAC. Modifications were made to the detector software to count the number of pulses and send a beam hold signal after a preset value. The detector system’s output was simply connected to the MLC interface board to induce the MLC hold-off signal with no complex circuitry needed. Dosimetric characterization of the beam was evaluated using Gafchromic films.
Results: The LINAC underwent the conversion. The detector system could resolve in real-time individual pulses, and the control system was implemented. Output measurements revealed a mean dose per pulse of 0.56 Gy/pulse corresponding to a mean dose rate of 168 Gy/s (for a PRF of 300 Hz) at the isocenter. PDDs showed a practical range of 2.5 cm.
Conclusion: An infrastructure for FLASH preclinical studies was established. A LINAC underwent conversion, and a pulse counting control system was attained. The control system enabled quantification of LINAC characteristics but needs to measure dose instead (as the dose per pulse was variable), which is the next task of this project.