ESTRO 38 Abstract book

S1068 ESTRO 38

breath-holds, and such patients exhibited higher reproducibility for the GTV position and duty-cycle efficiencies than patients treated in inspiratory phase. EP-1958 Eight different open face masks compatibility with surface guided radiotherapy M. Kügele 1 , E. Konradsson 1 , M. Nilsing 2 , S. Ceberg 3 1 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden ; 2 C-rad Positioning AB, Research and development, Uppsala, Sweden ; 3 Lund University, Medical Radiation Physics- Department of Clinical Sciences, Lund, Sweden Purpose or Objective Open face masks can be combined with optical surface scanning (OSS) for patient positioning and real time monitoring during radiotherapy treatment. For the open mask, the OSS system aims to detect the patient skin surface only and any unwanted signal from the mask might affect the OSS performance. An increasing number of mask materials are about to be introduced to the radiotherapy environment, but few studies have been carried out investigating the compatibility with OSS system. Also, in this study we had access to new masks which are not yet available on the commercial market. The aim of this study was to evaluate 1) eight different open face masks compatibility with the optical surface scanning system Catalyst TM (C-RAD Positioning AB, Uppsala, Sweden) and 2) the positioning accuracy using a novel surface algorithm for stereotactic radiosurgery (SRS). Material and Methods Eight open face masks from several vendors were molded onto a head phantom (Little Junior, Laerdal Medical, Orpington, UK). The OSS system automatically cropped away the mask and only used the skin surface for positioning. Markers were placed onto the skin surface of the phantom for CBCT evaluation purposes. Due to the different designs of the masks, the area of the surface visible for the OSS system varied (figure 1). The phantom was initially positioned on the treatment couch in an open mask at the reference position in isocenter and a CBCT was acquired to register the position. In order to test the novel SRS algorithms accuracy, an offset in the phantom position of 1 cm in all translational directions was introduced. The OSS system calculated couch shifts were sent over to the linac and was automatically shifted, using the auto couch function. A CBCT was acquired to verify the phantom position. For each mask, the marker position was evaluated using the Hounsfield unit profile extracted from the image registration in the treatment planning system (Eclipse 13.6, Varian Medical Systems, Palo Alto, CA). The position of the marker was evaluated in Matlab (Toolbox Release 2015b, The MathWorks, Inc., Natick, MA) in anterior-posterior (AP-PA), left-right (L-R), superior- inferior (S-I) position, respectively.

Results The range of the width and height of the masks were 6-12 cm and 12.5-29 cm, respectively, which resulted in different sized surfaces for the OSS system to use for the positioning calculation (figure 1). The median (range) position of the marker on the phantom surface for all masks were 0.0 (-0.1 - 0.1), 0.1 (-0.1 - 0.1), 0.1 (-0.1 - 0.5) mm in AP-PA, L-R and S-I directions, respectively (table 1). Conclusion Overall, all of the masks were compatible in combination with the OS system and the novel SRS algorithm. Regardless of the size of the skin surface, a high accuracy for surface based positioning using the novel SRS algorithm was observed. EP-1959 Performance of Marker-less Tracking for Gimbaled Dynamic Tumor Tracking M. Ziegler 1 , S. Lettmaier 1 , R. Fietkau 1 , C. Bert 1 1 University Hospital Erlangen, Radiation Oncology, Erlangen, Germany