ESTRO 36 Abstract Book

S760 ESTRO 36 2017 _______________________________________________________________________________________________

The study shows that for this specific value of kB studied and all the limitations of the model, the quenching effect will not significantly affect the scintillator output factor measurements in small 6 MV photon fields, and the quenching correction factor will be therefore close to the unity (u=1%). The impact of the stem signal (i.e. Cerenkov and fluorescence light produced in the optical fiber cable during irradiations) therefore remains the main influencing factor on such measurements. EP-1441 Repurposing of a small clinical x-ray source for radiobiology irradiations M. Barry 1 , R. Jones 1 , M. Fay 1 , D. Butler 2 , J. Lehmann 1 1 Calvary Mater Newcastle, Department of Radiation Oncology, Newcastle- NSW, Australia 2 Australian Radiation Protection and Nuclear Safety Agency, Radiation Therapy Section, Yallambie, Australia Purpose or Objective Around the clock availability of irradiation capability is desirable for creative design of radiobiology experiments. Clinical radiation systems are often only available after hours. Dedicated commercial cell irradiation systems are bulky and expensive. They might not be feasible due to financial or administrative constraints. This work reports on the repurposing of a retired clinical intraoperative kV irradiation system (Intrabeam™) for cell irradiations. Material and Methods The Intrabeam system is designed to deliver spherical dose distributions to surgical cavities. The in the adaption for cell irradiations, one of the supplied applicators has been modified to be fitted with custom collimators aiming to deliver a homogenous field across the cell dish. Several collimator designs have been tested and measured used using radiochromic EBT3 film. Additionally, measurements without a collimator were done in comparison and to support Monte Carlo simulations. Film calibrations were performed with national standard beams covering the energy range of the device. The BEAMnrc code and the 'NRC swept BEAM” source model have been used to characterize the dose distributions and to aid collimator development. Results Using the film measurements, the parameters of the Monte Carlo source model (swept angle and beam radius) were tuned to produce the final model. Very good agreement between measured and simulated dose profiles for the open source at 5, 7.5 and 10 cm distance from the tip was observed. (Figure 1)

Measurements with steel and aluminum collimator designs identified desirable characteristics for a suitable collimator: a long extension beyond the tip of the source and a diameter beyond the projected field size. Based on measurements and simulations, a cell culture plate irradiation rig has been designed and built, allowing for radiobiology experiments with different cell dishes and incorporating film measurements to verify dose delivery. (Figure 2)

Figure 2: Cell irradiation rig with Intrabeam, collimator and 96 well cell culture plate. Conclusion The repurposed x-ray system will allow for flexible irradiation of cell cultures for radiobiology experiments. Future plans include extension to small laboratory animal irradiations, as the unique design with the source of radiation being at the tip of an extended metal tube allows for high dose rates to small fields when in close proximity to the target. EP-1442 Fricke and Polymer gel dosimeters for radiotherapy pre-treatment 3D dosimetry G.M. Liosi 1 , L. Trombetta 2 , P. Salmoiraghi 2 , M. Mariani 1 , F. Locatelli 2 , E. Bombardieri 2 1 Politecnico di Milano, Energy- Nuclear Engineering Division, Milano, Italy 2 Cliniche Humanitas, Gavazzeni, Bergamo, Italy Purpose or Objective Pre-treatment dosimetry represents a fundamental step for the verification of radiation therapy outcome, and, in particular, an accurate and precise measurement of the 3D dose distribution with high spatial resolution has become of paramount importance. Aim of this work was the study and characterization of two gel-dosimetry systems (Fricke- and Polymer-gels), suitable for volumetric patient-specific 3D dosimetry. Fricke-gel dosimeters are based on the dose dependent oxidation of Fe 2+ ferrous ions –dispersed into a tissue equivalent gel matrix– into Fe 3+ ferric ions. Thus, the Fe 3+ concentration is linearly related to the absorbed dose. The MRI acquisition of gels through T1-weighted images permits measurement of Fe 3+ concentration, obtaining at the same time the absorbed dose mapping within the irradiated volume. On the other hand, as regard Polymer- gel dosimeters, a dose-dependent polymerization occurs, hence dose assessment and spatial information can be obtained by means of T2-weighted MRI analysis. Material and Methods A preliminary optimization of the chemical composition for both Fricke/Polymer- gel dosimeters was performed. Afterwards, the calibration method, MRI (1.5T) acquisition and reconstruction parameters were set for each system

Figure 1: Profiles of the dose distributions 10 cm from the open tip of the source (no collimator) as measured with EBT3 film and simulated using the BEAMnrc Monte Carlo code.