ESTRO 2020 Abstract book

S748 ESTRO 2020

PO-1327 Performances of new beam monitors based on Ultra Fast Silicon Detectors for proton therapy Z. Shakarami 1,2 , M. Donetti 3 , F. Fausti 2 , M. Ferrero 1,2 , S. Giordanengo 2 , O. Hammad Ali 1,2 , M. Mandurrino 2 , O.A. Marti Villarreal 1,2 , F.M. Milian 2,4 , V. Monaco 1,2 , R. Sacchi 1,2 , V. Sola 1,2 , A. Staiano 2 , A. Vignati 1,2 , R. Cirio 1,2 1 Universita Degli Studi di Torino, Physics department, Torino, Italy ; 2 INFN, National Institute of Nuclear Physics, Torino, Italy ; 3 CNAO, National Center of Oncological Adrotherapy, Pavia, Italy ; 4 Universidade Estadual de Santa Cruz, Department of Exact Sciences and technologies, Ilheus, Brazil Purpose or Objective To overcome many limitations of gas detector systems, such as reduced sensitivity and slow charge collection, sensors based on the Ultra Fast Silicon Detectors (UFSDs) technology are being developed. The main advantages of UFSDs are fast charge collection (~1 ns) and excellent time resolution (few tens of ps). A segmented UFSD sensor could allow discriminating and counting single protons up to high fluxes of therapeutic beams (10 8 p/s cm 2 ) and the time resolution could be exploited for measuring the proton beam energy, using time-of-flight (TOF) techniques. Material and Methods Two prototypes of strip detectors with different geometries and doping modalities, to improve radiation hardness, were developed. The sensor design for the first prototype was optimized to provide the possibility to count individual protons up to 10 8 p/cm 2 s, with an active thickness of 50 µm. A dedicated VLSI readout chip has been designed and produced to deal with a single proton signal of nanosecond duration and with 10 8 Hz signal rate on each channel. In the second prototype, UFSD sensors of 70 and 120 µm total thickness are positioned in a telescope configuration to allow the measurement of protons’ TOF (figure 1). To minimize the time-walk effect, the time of arrival of protons is obtained using a constant fraction discriminator algorithm. Through analytical approximation validated with Geant4 simulations, the corresponding beam energies are obtained from TOF values.

PO-1326 Evaluation of Percentage Depth Dose Measurement of High-energy Electron Beams using new TLD Sheet K. Sasaki 1 , Y. Shiota 2 , M. Miura 2 1 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan ; 2 Iwata City Hospital, Medical Physics, Iwata, Japan Purpose or Objective Although there are various devices for measuring high- energy radiation, Radiographic films, Radiochromic films, and parallel plate ionization chambers are generally used to obtain surface or build-up region doses with high accuracy. In this study, we measured the depth dose of high energy electron beams using a new TLD sheet dosimeter with an effective atomic number close to the water and high measurement flexibility and investigated the characteristics of electron beam measurement. Material and Methods Percentage depth dose (PDD) was measured using 4 MeV, 6 MeV, 9 MeV, and 12 MeV electron beams from Novalis TX (BrainLAB / Varian Medical systems) LINAC. Measurements were taken with a NACP-02 parallel plate ionization chamber (IBA), gafchromic films (Ashland) and thermoluminescence dosimeter sheets (TLD sheet, Toyo Medic Co., Ltd). The chemical formula of TLD sheet is LiB3O5, and its thickness is 150 micrometers, which is about half of the gafchromic film. Measurements using TLD sheets were performed in two ways. The first method was a parallel incidence method in which a sheet was sandwiched between plastic slab phantoms (WE211, Kyoto Kagaku) and a radiation beam was incident in parallel on the sheet. The second method was a stack method in which the radiation beam was incident vertically on the stacked TLD sheets. Results Depth scaling was performed using the density of the slab phantom in the parallel incidence method, and that was performed by the measured data of the TLD sheet in the stack method. The relationship between the measured digital value and the dose was linear, and the difference due to energy was very small. PDDs measured with ionization chambers or TLD sheets showed consistent data agreement at deeper depths than the maximum dose depth, but TLD sheets tended to be lower in the build-up region (Figure 1)

.This deviation in the stack method might be due to the uncertainty of the TLD sheet density obtained from the composition, and that in the parallel incidence method might be due to the gap between the slab phantom and the TLD sheet. Another possible factor is that the temperature uniformity of the reading system for the TLD sheet might have a certain slope toward the edge of the sheet. In addition, there might be energy dependence. Conclusion We evaluated the PDD of high-energy electron beams using a new material TLD sheet and confirmed its usefulness. The TLD sheet confirmed that the dose could be accurately evaluated at a depth deeper than the maximum dose depth. However, the build-up area was underestimated. This problem could be solved by improving the reading mechanism.