ESTRO 36 Abstract Book

S405 ESTRO 36 2017 _______________________________________________________________________________________________

Regarding MLC errors affecting the field size, large deviations from reference were observed in the IQM signal, while unidirectional shifts introduced deviations below detection limit. A similar behavior was observed for 2D γ and DVH parameters. Figure 1 illustrates the correlation of D mean (PTV) and IQM signal deviation, indicating that clinically relevant errors can be identified.

especially for personal dosimetry and phantom measurements. Reusability is one advantage of using OSL for dosimetry; the OSL particles can be reset by temperature or light-bleaching. Furthermore, the OSL material used in this study has a wide dynamic range and linear dose response, and the dosimeter matrix consists of a flexible material that can be cast into anthropomorphic 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

Conclusion The deviation between calculated and measured signal is relatively high, therefore a measurement should be defined as reference. With this limitation, the system is not yet capable of treatment plan verification but is a powerful tool for constancy testing. The detector provides excellent signal stability and is very sensitive regarding error detection. The signal deviation correlates with 2D γ and DVH metric deviations; this information can be used for identifying action limits for the IQM. PO-0773 Three-dimensional radiation dosimetry based on optically-stimulated luminescence M. Sadel 1 , E.M. Høye 2 , P. Skyt 2 , L.P. Muren 2 , J.B.B. Petersen 2 , P. Balling 1 1 Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark 2 Aarhus University Hospital, Department of Medical Physic, Aarhus, Denmark Purpose or Objective Modern radiotherapy employs complex 3D radiation fields to deliver therapeutic doses during treatment, and detailed quality assurance is a prerequisite. Methods based on luminescent passive detectors, such as optically stimulated luminescence (OSL), are widely applied,