  Radiotherapy is part of the treatment of over 50% of cancer patients. Its
efficacy is limited by the radiotoxicity to the healthy tissue. FLASH-RT is
based on the biological effect that ultra-high dose rates (UHDR) and very short
treatment times strongly reduce normal tissue toxicity, while preserving the
anti-tumoral effect. Despite many positive preclinical results, the translation
of FLASH-RT to the clinic is hampered by the lack of accurate dosimetry for
UHDR beams. To date radiochromic film is commonly used for dose assessment but
has the drawback of lengthy and cumbersome read out procedures. In this work,
we investigate the equivalence of a 2D OSL system to radiochromic film
dosimetry in terms of dose rate independency. The comparison of both systems
was done using the ElectronFlash linac. We investigated the dose rate
dependence by variation of the 1) modality, 2) pulse repetition frequency, 3)
pulse length and 4) source to surface distance. Additionally, we compared the
2D characteristics by field size measurements. The OSL calibration showed
transferable between conventional and UHDR modality. Both systems are equally
independent of average dose rate, pulse length and instantaneous dose rate. The
OSL system showed equivalent in field size determination within 3 sigma. We
show the promising nature of the 2D OSL system to serve as alternative for
radiochromic film in UHDR electron beams. However, more in depth
characterization is needed to assess its full potential.
