Study of the over-response of radiochromic films in ultra-high dose rate and dose per pulse electron beams

<i>Objective.</i>Many pre-clinical experiments on FLASH radiotherapy, in which biological samples are irradiated at conventional and ultra-high dose rate (UHDR), make use of radiochromic films as an absolute dose reference because their response is typically assumed to be dose rate independent. While several previous experiments confirmed this assumption, a recent study with protons found an over-response for Gafchromic EBT3 films at UHDR (Villoing<i>et al</i>2022<i>Med. Phys.</i><b>49</b>2732-45). If EBT3 is used as a reference in FLASH experiments, the over-response could result in an underdosing that falsely appears as sparing of normal tissue.<i>Approach.</i>To investigate whether the reported dose rate dependence of EBT3 can be reproduced with UHDR electron radiation as well, about 600 EBT3 samples were irradiated at the metrological linear accelerator facility at Physikalisch Technische Bundesanstalt, Braunschweig, and at the ELBE research accelerator at Helmholtz-Zentrum Dresden-Rossendorf, with different beam parameters. As dosimetric reference served a PTW flashDiamond detector which has been verified to show no deviation in the studied UHDR range as well as Physikalisch-Technische Bundesanstalt's alanine dosimetry system. In addition, several OC-1 films, which have been shown to be dose rate independent in previous studies, were irradiated together with EBT3 films.<i>Main results.</i>EBT3 shows a significant over-response at ultra-high dose per pulse and intra-pulse dose rates. Up to 3 Gy per pulse no significant deviations were observed. However, for larger doses per pulse an over-response was observed that increases with dose per pulse to up to 50%. For OC-1 films no significant effects were found.<i>Significance.</i>We provide evidence for the over-response of EBT3 films in UHDR electron beams. Therefore, EBT3 films should be used with caution in FLASH experiments.