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FLASH RT: Current Status of Dosimetry, QA , Challenges and the Need for Further Developments
Ultra-high dose-rate (UHDR) irradiations, known as FLASH radiotherapy (RT), rely on delivery of therapeutic doses at instantaneous dose-rates over four orders of magnitude higher than those currently used in conventional radiotherapy. It has been shown that such an extremely short delivery of radiation leads to remarkable reduction of normal tissue toxicity with respect to conventional dose-rate RT. Given the size of the error of biological contributions in radiotherapy outcomes, it is important that the physical errors are minimized. According to ICRU Report 24 a change of 7-10% in dose to target volume results in clinically significant change in tumour control probability. Therefore, it has been agreed that there is a requirement for an accuracy of ± 5% (k=2) in the delivery of absorbed dose to a tumour volume. These dosimetric constraints are fulfilled in conventional radiotherapy, where dosimetry methods and protocols have solid foundations. However, dosimetry at ultra-high dose-rate is notoriously complicated and it is essential to understand the effects that will influence detector response. To date, FLASH RT research has been focused on finding pragmatic solutions that allow the use of UHDR beams in the research setting, but there has been limited focus on accurate dosimetry and robust quality assurance (QA) for FLASH RT. There are limited data on the functionality of existing standard dosimeters when they are used to measure beams delivered in UHDR mode, hence it is essential to evaluate whether these dosimeters are appropriate and to report their sensitivities and associated uncertainties when used in UHDR regime. Given that the FLASH RT field is moving forward very fast, with the first clinical trials already commenced, it is of the utmost importance to ensure safe delivery of this new treatment modality to the patients. Without a clear understanding of the fundamental dosimetry issues, there is potential for significant dosimetric errors. One of the examples of dosimetric challenges that need to be approached in FLASH RT, are non-negligible ion recombination effects exhibited by ionization chambers (used as golden standard in conventional RT), when operated at UHDR regime. These challenges cannot be addressed and solved by a single institute or a vendor, hence the collective research efforts are needed with the involvement of National Measurement Institutes (NMIs), whose role is to underpin accurate measurements.
This lecture will address the aspects of reference dosimetry and QA in the clinical environment and highlight the challenges associated with the implementation of the established techniques in FLASH RT mode. Moreover, the UHDpulse, a joint research project within the European Metrology Programme for Innovation and Research will be highlighted, which aims to develop the metrological tools needed to establish traceability in absorbed dose measurements of UHDR particle beams.