ESTRO 2023 - Abstract Book

S779

Monday 15 May 2023

ESTRO 2023

deviation) of 0.25MeV. The effect of substituting the surrounding Styrofoam for graphite to determine both correction factors was also examined. Results Provisional (without modelling core thermistors) k_imp and k_gap correction factors where the Styrofoam had not been substituted were calculated to be 1.0014 ± 0.0001 and 1.0006 ± 0.0001 respectively. The graphite substituted Styrofoam geometry gave 1.0057 ± 0.0001 and 0.9997 ± 0.0001 respectively. All quoted uncertainties are Type A. These values show reasonable agreement between the k_gap correction factors in the two configurations. However, there is a large discrepancy between the values determined for the k_imp correction factors. Conclusion Provisional simulations show that modelling the surrounding Styrofoam material of the SPGC as graphite leads to a significant increase in the k_imp correction factor. This is due to the increased amount of scatter into the core, and therefore dose, from the surrounding material. Separation of the scattering component into a new correction factor, k_scat, may be required. However, preliminary analysis shows agreement within 0.2% of the primary standard after applying these corrections. OC-0932 Plastic scintillator-based dosimeters for FLASH radiotherapy E. Ciarrocchi 1,2 , M. Morrocchi 1,2 , D. Del Sarto 3,2 , F. Di Martino 4,2 , S. Linsalata 4 , J.H. Pensavalle 3,2 , M.G. Bisogni 1,2 1 University of Pisa, Department of Physics, Pisa, Italy; 2 Istituto Nazionale di Fisica Nucleare, Pisa Section, Pisa, Italy; 3 University of Pisa, Scuola di Specializzazione in Fisica Medica, Pisa, Italy; 4 Azienda Ospedaliero-Unversitaria Pisana, U. O. Fisica Sanitaria, Pisa, Italy Purpose or Objective This contribution describes the development, for the INFN project FRIDA (Flash Radiotherapy with hIgh Dose-rate particle beAms), of dosimeters based on plastic scintillating fibers imaged by a CCD camera, and their performance evaluation by means of irradiations with the electron FLASH research accelerator located at the Centro Pisano Flash Radiotherapy (CPFR) in Pisa. Materials and Methods The first dosimeter was composed of a 10-mm long plastic scintillator fiber, with 1-mm diameter, coupled to a 25-m long clear optical fiber, whose output was imaged by a CCD camera. Cyanoacrylate glue was used for coupling, which was performed in a peek opaque tube for mechanical stability. The dosimeter was embedded in a PMMA support and placed in a solid water phantom, irradiated with an electron FLASH triode-gun accelerator (Fig.1). Two applicators were used, with 100-mm and 40-mm diameter, respectively. The beam energy was fixed at 9 MeV, and the following beam parameters were varied to test the dosimeter dependence on them: 1) dose per pulse in the range 30-760 cGy/p; 2) pulse repetition frequency (with fixed dose per pulse, pulse length and number of pulses), thus changing the total irradiation time and average dose rate; 3) intra-pulse dose rate (changing also the pulse length in order to have a fixed dose per pulse of 4 Gy/p). The Cerenkov radiation contribution was also estimated by irradiating a portion of the clear optical fiber in place of the scintillating fiber. A second, smaller dosimeter was also developed, with 3 mm length, 500 um diameter, coupled to a 10-m long clear optical fiber with 250 um diameter. In this case, epoxy was used for optical coupling inside a black carbon tube. This prototype will be tested in the future data taking sessions.

Results Preliminary results show that the first dosimeter exhibits the following properties (Fig. 2): 1) only modest saturation at high dose per pulse values in the investigated range of 30-760 cGy per pulse, with a Birks’ coefficient of 2.4e-4 AU/MeV using the fit function: S = a*E/(1 + b*E); 2) independence from the pulse repetition frequency, with less than 1% fluctuations in the investigated range of 5-245 Hz; 3) slight dependence on the intra-pulse dose rate, with a maximum variation of approximately 7% increasing the intra-pulse dose rate from 1 to 4 MGy/s; 4) less than 15% of the detected light can be due to Cerenkov radiation.