344 - Ultrahigh Dose Rate Electron Beams from a Compact X-Band Photoinjector Linac Reaching Energies up to 70 MeV: A Path to Electron FLASH Radiotherapy for Deep Tumors

Purpose/Objective(s): The potential for increased relative normal tissue sparing through the FLASH effect may markedly improve the therapeutic window of radiation therapy. Investigations into the clinical translation of this effect are predominantly limited by the few ionizing radiation source architectures capable of producing sufficient dose rates, generally > 40 Gy/s. For electron FLASH, compact sources have been limited to energies up to 22 MeV, resulting in insufficient penetration for deep tumors. While one S-band linac has demonstrated the FLASH effect at electron energies up to 250 MeV, the large facility-scale architecture limits potential for clinical translation. Here we investigate the use of a compact X-band Radiofrequency (RF) photoinjector linac, driven by RF-matched, high-repetition rate laser pulse trains in a distributed charge architecture, to reach dose rates > 40 Gy/s with energies up to 70 MeV.

Materials/Methods: Electron beams were generated using an approximately 3-meter-long X-band RF photoinjector linac. Electron energy was measured using a dipole magnet spectrometer. The electron beam current was measured throughout the beamline, and electrons were propagated into air through a diamond vacuum window to minimize charge loss. The beam was shaped into a flat field using a scatterer-collimator scheme and subsequent dosimetry was conducted using radiochromic film. Films were analyzed using a three-channel digital scanner. Mean dose rates were calculated based on the number of electron pulses selected per treatment and their measured pulse widths and repetition rates.

Results: Electron energies of 25, 45, and 71 MeV were investigated. Flat circular fields were generated with diameters ranging from 0.8 cm to 1.7 cm with delivered surface doses ranging from 5-40 Gy across experiments. Field-averaged mean surface dose rates exceeded 40 Gy/s under all tested configurations, with a maximum field-averaged dose rate of 116 Gy/s with a 23-Gy irradiation achieved at 71 MeV using a 0.8-cm field.

Conclusion: These initial results demonstrate that an X-band RF photoinjector linac architecture, when driven by an RF-matched laser pulse train, can achieve ultrahigh dose rates at electron energies not accessible by conventional linacs. This platform enables preclinical investigation of high-energy electron FLASH and supports further development toward clinically relevant depths for deep-seated tumors.