ESTRO 2022 - Abstract Book

S1320

Abstract book

ESTRO 2022

Conclusion The dose response reported for larger samples measured using the optical CT scanner operating at 635 nm was greater than that for cuvettes as reported by spectrophotometry at 625 nm. Further studies are needed to determine whether the difference is related to variations in volume, shape, readout modality or irradiation conditions. Both optical CT of large samples and spectrophotometry of small samples provide high-quality relative measurements of absorbed dose. However, careful calibration is needed to relate these two distinct types of experiments.

PO-1539 In-silico assessment of the out-of-field over-response of an optically stimulated luminescent film

M. Caprioli 1 , M. De Saint-Hubert 2 , L. de Freitas Nascimento 3 , L. Delombaerde 4 , S.S. Galvez Febles 5 , K. Himschoot 6 , D. Vandenbroucke 6 , P. Leblans 6 , W. Crijns 7 1 KU Leuven, Oncology, Leuven, Belgium; 2 Belgian Nuclear Research Centre (SCK CEN), Research in Dosimetric Application, Mol, Belgium; 3 Belgian Nuclear Research Centre (SCK CEN), Research in Dosimetric Application , Mol, Belgium; 4 KU Leuven , Oncology, Leuven, Belgium; 5 Università degli Studi di Milano, Fisica, Milano, Italy; 6 Agfa N.V., Corporate Innovation Office, Mortsel, Belgium; 7 University Hospital Leuven, Department of Radiation Oncology, Leuven, Belgium Purpose or Objective The QUARTEL project aims to optimize 2D Optically Stimulated Luminescent (OSL) film for pre-treatment radiotherapy dosimetry. This dosimeter has an energy dependent signal resulting in a varying response in function of position (in-field vs out-field), beam-modulation (static beams vs IMRT), etc. This work investigates in-silico evaluation of the out-of-field over- response to avoid measurements-based assessment using existing analytical Z-effective calculations (Z eff C) and Monte Carlo (MC) dose simulations. Materials and Methods In the context of the QUARTEL project three OSL films were produced utilizing a BaFBr:Eu 2+ powder embedded in a water equivalent polymer-based binder. Three samples with different weight concentration were tested: 5,2% (A), 6.5% (B), 10% (C). For the in-silico assessment, these phosphor films were first modeled as homogeneous distributions of BaFBr:Eu 2+ . The OSL over-response assessment uses (1) a complete out of field energy spectrum (setup1: Varian 6MV, 10cm off-axis, 10x10 cm 2 field, 0° gantry angle, 90 cm SSD, 5cm depth) and (2) two mono-energetic spectra representing the in-field (1.25MeV) and out-of-field (0.115 MeV) situations for both Z eff C and MC simulations (MCNP 6.2.). The over-response in setup 1) was measured in a Varian Linac and considers the mean value of a 1x1 cm 2 ROI at 7cm from the central axis due to the limited size of the film. All the data (Z eff , dose) were divided by the corresponding response in water as in (1) and (2). Results The measured over-response is consistent with the in-silico assessment (Figure 1 and 2), although the difference between samples A and B is comparable with the standard deviation within the ROI: 0.0069. The calculated Z eff is 1.48 (A), 1.53 (B), and 1.77 (C) times higher than water. Similarly, for MC, the simulated dose response increases with the phosphor (%) from 1.77 (A) to 2.45 (C) with a precision of 0.29%. The in-silico results passing from the represented in-field to the out-of-field regions (2) show an increased over-response at lower energies (Table 1) up to 2.41 times for the MC simulated sample C.