BACKGROUND: FLASH radiotherapy using megavoltage (MV) photon beams should enable greater therapeutic efficacy, target deep seated tumors, and provide insights into mechanisms within FLASH. PURPOSE: In this study, we aim to show how to facilitate ultra-high dose rates (FLASH) with MV photons over a field size of 12-15 mm, using a 6 MeV (nominal) preclinical electron linear accelerator (linac). Our intention is to utilize this setup to deliver FLASH with MV photons in future preclinical experiments. METHODS: An electron linear accelerator operating at a pulse repetition frequency of 300 Hz, a tungsten target, and a beam hardening filter were used, in conjunction with beam tuning and source-to-surface distance (SSD) reduction. Depth dose curves, beam profiles, and average dose rates were determined using EBT-XD Gafchromic film, and an Advanced Markus ionization chamber was used to measure the photon charge output. RESULTS: A 0.55 mm thick tungsten target, in combination with a 6 mm thick copper hardening filter were found to produce photon FLASH dose rates, with minimal electron contamination, delivering dose rates >
40 Gy/s over fields of 12-15 mm. Beam flatness and symmetry were comparable in horizontal and vertical planes. CONCLUSION: Ultra-high average dose rate beams have been achieved with MV photons for preclinical irradiation fields, enabling future preclinical FLASH radiation experiments.