Dosimetry for FLASH and other non-standard radiotherapy sources

We review current dosimetry practices for non-standard radiotherapy sources. We classify radiotherapy sources as established, variations of established, or novel. Our review concentrates on novel sources including ultra-high dose-rate (FLASH) sources and some that have yet to be used for clinical radiotherapy. Factors which differentiate non-standard sources include dose-rate, temporal pulse structure, spatial fractionation, focussing, the presence of magnetic fields, and energy range. For the most part we exclude techniques which use materials inside the tumor to modify the dose. Dosimetry techniques include ionization chambers, film, alanine, calorimetry, and solid-state detectors. We review dosimetry only, neglecting other issues such as beam monitoring, patient delivery systems and treatment planning.