ESTRO 36 Abstract Book

S410 ESTRO 36 _______________________________________________________________________________________________

shapes and simulate organ deformations during RT. In this abstract we propose a new, reusable 3D dosimetry system based on OSL material embedded homogenously inside a transparent matrix. Material and Methods Cuvette-sized prototypes of the dosimeter were produced, consisting of a matrix; 4 g of a transparent silicone elastomer (SE) (Sylgard 184, Dow Corning), and a homogeneously embedded OSL material; 0.3 g of lithium fluoride (LiF) doped with magnesium, copper and phosphorus (LiF:Mg,Cu,P - MCP). Three samples were prepared in standard OSL-reader aluminum trays; a reference sample with silicone elastomer, and two samples with OSL powder embedded in the SE matrix, containing 0.06 mg and 0.2 mg OSL powder (sample 1 and 2 respectively). They were read-out using a Risø TL/OSL DA-20 reader. Samples were irradiated with 1 Gy beta radiation and stimulated for 100 s with blue light emitting diodes (LEDs), with emission centered at 470 nm and an intensity of ~80 mW/cm 2 . Results The transparency of the dosimeter (see Fig. 1) depended on the concentration of MCP powder, which must be optimized as a compromise between signal level per volume and overall transparency. The refractive-index match between LiF and the SE is quite good for visible wavelengths, which minimizes light scattering from the particles. Approximately 10,000 and 40,000 counts were detected in 1 second per 1mm 3 voxel from samples 1 and 2, respectively, corresponding to the anticipated signal levels. Also, the silicone matrix in itself did not add to the OSL signal (see Fig. 2). 3D distributions can be obtained without the need for inversion algorithms, for example, by stimulating the OSL dosimeter with a light sheet (from a laser source), and imaging the luminescence intensity across that sheet (by a combination of optical filters and a camera), and shifting this plane across the dosimeter. Conclusion A new 3D dosimeter system based on OSL material has been presented. It has the potential to verify complex 3D RT doses with high spatial resolution, while maintaining the advantages known from personal-dosimetry use of OSL. PO-0774 Investigation of dose-rate dependence at an extensive range for PRESAGE radiochromic dosimeter E.P. Pappas 1 , E. Zoros 1 , K. Zourari 2 , C.I. Hourdakis 2 , P. Papagiannis 1 , P. Karaiskos 1 , E. Pantelis 1 1 National and Kapodistrian University of Athens, Medical Physics Laboratory - Medical School, Athens, Greece 2 Greek Atomic Energy Commission, Division of Licensing and Inspections, Athens, Greece Purpose or Objective The purpose is to investigate dose-rate dependence effects for a recent formulation of the commercially available PRESAGE radiochromic dosimeter (Heuris Inc, NJ) in a wide range of dose delivery rates extending to three orders of magnitude (0.018 – 19 Gy/min). Material and Methods In order to achieve an extensive dose rate range, this work was divided into two separate studies. Lower dose rates were delivered by 60 Co beams while higher dose rates were achieved by a flattening-filter-free (FFF) linear accelerator. For the low dose rate part of this study, 10 PMMA cuvettes (1×1×4 cm 3 ), filled with PRESAGE samples, were irradiated to the same dose with 5 different dose rates. Irradiations were performed with a 60 Co PICKER unit in a secondary standard calibration laboratory. The samples were divided into groups of two and each group was placed at a different distance (56.65 - 427 cm) from the 60 Co source at a 5cm depth within a water phantom. Irradiation times varied in order to deliver the same dose of 1 Gy at the center of all cuvettes with dose rates in the

range of 0.018 – 1.0 Gy/min. For the high dose rate study, a similar methodology was employed. Four couples of PRESAGE cuvettes were placed within a slab in a solid water phantom and irradiated at different dose-rates by varying the dose delivery rate of an ELEKTA Versa HD FFF linac from 2.5 up to 19 Gy/min. Dose delivery of 1 Gy for all dose rates was verified by ion chamber measurements. Irradiation induced optical density (OD) change was measured from pre- and post-irradiation scans with a digital spectrophotometer operated at 633 nm. Mean OD change for each group was normalized to the value for the highest dose rate in each study. Results Results presented in figure 1 show a trend of increasing PRESAGE dose sensitivity with decreasing dose rate with the over-response reaching up to 16% at 0.018 Gy/min. Although in a first approach such low dose rates could be considered extremely low in external radiotherapy, recent studies have shown that in advanced radiotherapy techniques (e.g. VMAT) dose rate varies drastically across dose distributions delivered and a considerable contribution of the delivered dose could come from very low dose rates (0.01 - 0.1Gy/min). Regarding the high dose rate study, all responses agree within experimental uncertainties, indicating that PRESAGE sensitivity is not significantly affected. Figure 1: Dose rate dependence of PRESAGE response for both studies included in this work. Error bars correspond to 1 standard deviation of all experimental uncertainties involved. Conclusion Results of this study indicate a significant over-response of this PRESAGE formulation in very low dose rates that should be considered when they are used in applications involving wide range of dose delivery rates. Acknowledgement: This work was financially supported by the State Scholarships Foundation of Greece through the program ‘Research Projects for Excellence IKY/SIEMENS’. PO-0775 Contributions to detector response in arbitrary photon fields S. Wegener 1 , O.A. Sauer 1 1 University Hospital, Radiation Oncology, Würzburg, Germany Purpose or Objective Due to their small active volumes, diodes are often the detectors of choice for many commissioning tasks including the measurement of output factors, especially in small fields. However, high-atomic number material in the chip, detector shielding or other components and a finite active volume size have been found to alter the signal compared to the dose ratios measured in water in the absence of such a detector. As a consequence, correction factors need to be applied to correct the obtained signals. Using three experimental setups (fig. 1), the different contributions to the detector signals were separated and analyzed: the response to scatter, the primary beam and the combination of both. Material and Methods Signal ratios were obtained for three different experimental setups (fig. 1): First, the standard open field geometry. Secondly, fields in which the central part of the beam was blocked out by a 4 mm aluminum pole and the