ESTRO 2023 - Abstract Book

S1463

Digital Posters

ESTRO 2023

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). Results 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

Conclusion 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.

PO-1743 A novel anthropomorphic surrogate for advanced QA in stereotactic adaptive radiotherapy deliveries

M. Dierl 1 , M. Robinson 2 , M. Rose 2 , A.A. Schönfeld 3 , N. Chofor 3

1 Klinikum Bayreuth GmbH, Department of Radiation Therapy, Bayreuth, Germany; 2 Sun Nuclear, A Mirion Medical Company, Product Strategy, Melbourne, USA; 3 Sun Nuclear, A Mirion Medical Company, Research and Development, Melbourne, USA Purpose or Objective Adaptive radiotherapy (ART) quality assurance (QA) requires the utilization of phantoms and detector combinations capable of tracking and accounting for deviations from the initially planned treatment configuration. To enhance the capabilities of the SRS MapCHECK in ART QA for stereotactic deliveries, a new add-on surrogate has been tested for the StereoPHAN. In this study, we investigated the surrogate’s (i) influence on the dose distribution within the treatment planning system (TPS) and the dosimetric verification with the SRS MapCHECK, (ii) detectability of the surrogate and its use in combination with a VisionRT surface-guided radiotherapy system and (iii) its usage in combination with the Dynamic Platform model 008PL (Sun Nuclear, A Mirion Medical Company, Norfolk, USA) to enhance gated dose deliveries. Materials and Methods The anthropomorphic surrogate under investigation, see Figure 1, was designed to fit onto the StereoPHAN, which typically houses the SRS MapCHECK (both Sun Nuclear, A Mirion Medical Company, Melbourne, USA). CTs were obtained for the same setup with and without the anthropomorphic surrogate in place to assess adequate density overwrites for the TPS. Typical clinical plans comprising of an open static field and an open arc, each of 4 × 4 cm ² , were delivered and investigated for both setup configurations, with results evaluated in SNC Patient. Next, the capability of the system to enhance surface guidance deliveries was evaluated in combination with a VisionRT system. Following prior setup using the XVI imager, the misalignment of the hexapod tabletop at a Versa HD linac (Elekta AB, Stockholm, Sweden) was tracked using the VisionRT