Session Item

Significant dosimetric challenges should be dealt with ultra-high dose rate (UHDR) beams for FLASH radiotherapy. Reference dosimetry at UHDRS has not yet fully established and many efforts are made to develope alternative techniques and detectors, as ionization chambers suffer of ion recombination effects. Alanine passive dosimeters still represent a reliable reference for UHDR measurements, although dose determination is typically time consuming. This work aims at investigating the response of alanine pellets exposed to UHDR electron beams to comprehensively assess this dosimetric approach as a reference method for absolute dose measurements at UHDRs. Dose rate systematic investigations have been also carried out. Moreover, a multi-centre cross-comparison was performed, to check consistency of the measurements at the same irradiation conditions, using different alanine batches and independent readout systems.

Alanine pellets (produced by Gamma Service GmbH) were irradiated with ultra-high dose rate (UHDR) electron beams at 7 and 9 MeV using the SIT Sordina ElectronFLASH linac at conventional and UHDR regimes were used. Pulse structure is characterized by a pulse duration between 1-4 μs and a frequency up to hundreds of Hz. High average dose rates (up to MGy/s) were adopted for the experimental campaign, characterized by instantaneous dose rate even more than two orders of magnitudes larger. PMMA phantoms were used for placing the alanine pellets at the same relative positions. Monte Carlo simulations were carried out through the GEANT4 toolkit in order to model the response of alanine pellets. The response of alanine pellets was compared with that of a FLASH diamond detector (whose response was found to be independent of dose rate also for UHDR regimes).  

We carried out measurements at different dose rates spanning a few orders of magnitudes and investigating possible effects of the polymerization rate in the PDDs, which were acquired stacking the alanine pellets into a specifically designed PMMA phantom. We have found a good agreement between experimental measurements and MC calculations for 7 MeV electron beams (as shown in Figure 1a).

We also found a good agreement (within a few percents) between the dose reconstructed with the alanine pellets and those measured by means of a FLASH diamond active detector up to about 7 Gy/pulse.

      

Figure 1: Left a) Comparison of percentage depth dose profile obtained through alanine pellets and Monte Carlo simulations. Right: Comparison of reconstructed dose values with alanine pellets and FLASH diamond detector.

Finally, we found response of the batches from different institutions was comparable within the overall uncertainties

Alanine dosimeters were found to have a response independent of dose rate for UHDR regimes and therefore are suitable for reference dosimetry of UHDR beams used for FLASH radiotherapy.