Ion Recombination in Ultra-Short High Dose-Per-Pulse Very High Energy Electrons (VHEEs)

Recent Monte Carlo (MC) studies have shown that the use of Very high Energy Electrons (VHEEs), with energies up-to 250 MeV, can provide more favourable dose distributions in comparison to current photon and electron therapy. Ionisation chamber response and correction factors, in particular for ion recombination, are typically small at clinical energies and can be accounted for with high precision according to an electron beam dosimetry Code of Practice. In high dose-per-pulse beams, ion recombination is a larger concern with a correction of approximately 20% being observed for Intraoperative Radiotherapy (IORT) beams. For the case of ultra-short VHEEs the dose-per-pulse can be orders of magnitude higher than that of IORT. Therefore, the ability to correct accurately for ion recombination will play a crucial role in dose determination using ionisation chambers. Current recombination models are quoted to be accurate for the high saturation region where more than 70% of the charge is being collected by the chamber. Those models are expected to be invalid for the ultra-short VHEEs and a new ion recombination correction procedure may be required.