ESTRO 36 Abstract Book

S769 ESTRO 36 _______________________________________________________________________________________________

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 to optimize the dose sensitivity and the Signal-to-Noise Ratio. In particular, geometrical distortion, image homogeneity, artifacts, image texture, dose accuracy and resolution, limit of detectability (LOD) and quantification (LOQ), Fe 3+ spatial diffusion (Fricke-gels) and dose rate dependence were evaluated. Finally, a pre-treatment dosimetry of a SBRT plan was acquired and a relative planar profiles comparison with a standard dosimeter (Gafchromic EBT2) was performed. Ad hoc Matlab codes were developed for images analysis. Results The chemical composition, MRI acquisition and reconstruction parameters were optimized for each gel system. No image correction maps were needed, since geometrical distortion, artifacts and inhomogeneity were always negligible, and no dependence on photon beam dose rate was observed. 3D spatial resolution (voxel dimension) was 1x1x3mm 3 . Dose accuracy was under 4% in the range 18-25Gy, but worst for lower doses. Dose resolution was about 1Gy, while LOD was less than 0.5Gy. Differences between gel systems and Gafchromic profiles’ FWHMs were in the range 0,5mm – 5,5mm, mean dose deviations in flat region were always around 2%, while penumbra differences were about 2mm. Negligible diffusion and time effects were observed up to 3 hours from irradiation for all gel systems. Conclusion This study showed that both Fricke/Polymer- gel dosimeter could be a suitable tool to perform pre- treatment QA, with particular focus on SBRT and SRS treatments, thanks to their optimal spatial resolution, their practicability and their capability to perform 3D dosimetry. Further studies are ongoing to standardize a protocol to perform 3D pre-treatment dosimetry. EP-1443 Measurement of 3D dose distributions from an MR Linac with gel dosimetry Y. Roed 1,2 , L. Pinsky 1 , G. Ibbott 2 1 University of Houston, Physics, Houston, USA 2 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA Purpose or Objective To demonstrate the potential value of polymer gels to measure 3D dose distributions delivered with an MR-image guided radiotherapy delivery machine.

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. 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