Characteristics of portable primary standards for dosimetry in clinical UHDR proton beams
All portable standards need characterisation for the beam used for the measurements. FLASH-DOSE will go beyond the state of the art by characterising and optimising one portable water calorimeter and two portable graphite calorimeters for use in scanning UHDR proton beams, with a target uncertainty 0.5 % – 0.9 % (k = 1) depending on the standard. The uncertainty budgets for these standards will be evaluated in a direct comparison of the three standards in the same facility. This will lead to the first Dw measurements applying a water calorimeter in a scanning UHDR proton beam. The comparison will, for the first time ever, provide a degree of equivalence of the different standards in scanning UHDR proton beam and will be a significant step forward for dosimetry in proton beams in general, and FLASH dosimetry, in particular.
Determination of secondary standard correction factors by developing dedicated simulation models
The project will progress beyond the state of the art by simulating recombination effects to calculate ks in scanning UHDR proton beams and UHDPP electron beams using more realistic models of the ionisation chamber geometry, including free electrons and space charge effects and based on measured input data. The second output of these recombination simulations will be practical formalisms for the determination of ks. Using these simulation models the accuracy of existing more sophisticated practical formalisms will be evaluated. Furthermore, the sensitivity of the larger ks corrections to influence quantities and to ion chamber dimensions will be investigated, so that uncertainty budgets for simulated ks and for ks determined from practical formalisms can be evaluated. In particular for fast ultra-thin ionisation chambers, improved modelling should lead to a better description of ks for these novel chambers.
The project will also progress beyond the state of the art by investigating the impact of field sizes, and field flatness on kQ for scanning UHDR proton beams using Monte Carlo radiation transport simulations, so that in cases where reference conditions in UHDR scanning proton beams require smaller field sizes the impact of these influence quantities can be included (see Obj. 3). In addition, the project will go beyond the state of the art by simulating the impact of enhanced dose inhomogeneity on kQ, due to different collimation methods, in reference UHDPP electron fields. Furthermore, safety and repeatability limitations in dose delivery can potentially lead to increased uncertainty in the determination of the secondary standards reference position for UHDPP electron beams. Therefore, the impact of this increase on the uncertainty of kQ and for an alternative reference position (R100) will be investigated.
Reference dosimetry methodology for clinical UHDR proton therapy facilities
The project will progress beyond the current state of the art by developing a reference dosimetry methodology for the determination of Dw with secondary standards in UHDR scanning proton beams. The target uncertainty is similar to that obtained in conventional proton beams (1.7 %, k = 1).
Crucial for this methodology is the definition of a set of reference conditions suited for clinical scanning UHDR proton beams. These reference conditions will be defined using IAEA TRS-398 as a starting point with extensions accounting for (i) the influence of the different temporal beam structure and instantaneous dose-rate, (ii) other influence quantities (e.g. field size and flatness), and (iii) what can practically be realised with the technology currently available at clinical scanning UHDR proton facilities. To verify the developed reference conditions in scanning UHDR proton beams and to determine the beam quality specifier, the project will develop methodology to measure relevant beam characteristics of these beams. Using the developed reference conditions kQ and ks will be independently determined experimentally in different facilities for a set of secondary standards and compared with simulated data, and, for ks, with the results from the investigated practical formalism. The sensitivity of ks and kQ to (additional) influence quantities will be used to evaluate their uncertainty contributions and the impact of potential divergence from the IAEA TRS-398 reference conditions. Finally, the defined set of reference conditions will be validated by demonstration of the consistency of the full traceability routes A and B and with a passive dose-rate independent dosimeter.
The measured kQ will include results for novel detectors for which no kQ data currently exists. This dataset and the dataset for ks will include a detailed uncertainty budget for the determination of Dw using the investigated secondary standards, which allows the assessment of the suitability of these detectors for reference dosimetry in scanning UHDR proton beams.
Reference dosimetry methodology for clinical UHDPP electron therapy facilities
In contrast to the project’s approach for scanning UHDR proton beams based on reference conditions, for UHDPP electron beams, the project will use existing UHDPP electron reference fields at NMIs and optimise them, to mimic the reference conditions of clinical electron FLASH accelerators. This will allow direct calibration of secondary standards in beams with properties close to clinical UHDPP electron beams thereby introducing a new traceability route. Novel and existing secondary standards will also be characterised for recombination in UHDPP beams and their kQ factors will be determined, taking into account the characteristics of the new clinical electron UHDPP facilities. Alternative more efficient procedures for measurement of beam characteristics and beam quality specifiers based on novel detectors suited for UHDPP conditions will be developed. The consistency of traceability routes will be investigated, and finally, recommendations for the determination of Dw under reference conditions in UHDPP electron fields will be drafted with a target uncertainty similar to uncertainties in conventional electron beams (1.2 %, k = 1, for plane parallel ionisation chambers).
Progress beyond 18HLT04 UHDpulse
The completed EMPIR project 18HLT04 UHDpulse represents in many ways, the current state of the art in dosimetry for FLASH radiotherapy. The focus in 18HLT04 UHDpulse was on primary standard development for UHDDP electron beams and fundamental development of new detectors such as the flashDiamond and ultra-thin ionisation chambers. This project will develop methodology for traceable dosimetry in support of CoP for clinical FLASH radiotherapy facilities by building on the findings of 18HLT04 UHDpulse. The 18HLT04 UHDpulse project did not cover the scanning UHDR proton beams which are part of WP1 and WP3 in this project. In addition, this project will develop reference dosimetry for the new clinical UHDPP electron beams using novel detectors by providing traceably measured correction factors.