ESTRO 2020 Abstract Book

S870 ESTRO 2020

rotation offsets obtained by a Winston-Lutz (WL) test using the MV panel. Results Mean translational and mean rotational isocenter shifts of the non-coplanar treatment for the population of 7 volunteers are (0.24±0.09) mm and (0.15±0.07) degrees, respectively, averaged over the couch angles. Isocenter shift values larger than 0.5mm were found for couch angles 45˚ and 90˚ (fig.1.). These results can be correlated with the couch rotation offsets obtained by regularly repeated WL tests: mean vector deviations between the couch and radiation isocenters at gantry 0° are 0.36±0.15, 0.24±0.09, 0.30±0.14, 0.59±0.11 and 0.60±0.13 (mean±SD in mm) for couch rotations 0°, 315°, 270°, 90° and 45°, respectively, revealing isocenter shift values larger than 0.5 mm for couch rotations 90° and 45°.

Conclusion These preliminary results show that RayPilot may be a viable method for tumour tracking during prostate SBRT alongside other imaging modalities. The positional differences recorded between the device and the CT and CBCT images requires further investigation before the RayPilot device could be considered as the stand-alone modality for positional verification in SBRT in our department. PO-1599 Implementation of a commercial optical surface tracking system for non-coplanar SRS treatments A. Swinnen 1 , M. Öllers 1 , F. Verhaegen 1 1 MAASTRO Clinic, Radiotherapy department, Maastricht, The Netherlands Purpose or Objective Using a non-coplanar technique, the coincidence between radiation and couch rotation isocenter has to be as accurate as possible as one is generally not able to correct for such errors during treatment delivery. Further, immobilization masks cannot fully eliminate intra- fractional movements of a patient. The aim is to evaluate the feasibility of a commercial optical surface tracking (OST) system to monitor patient positioning during non- coplanar stereotactic radiosurgery (SRS) treatments. Material and Methods A dedicated 3-camera OST system was used (Catalyst HD TM , C-RAD) on a Varian Truebeam STx linac with a 6DoF couch. The OST system projects LED light of 3 wavelengths onto the patient and a charge-coupled device camera to detect the reflected light from the patient, to generate a live 3D surface of the patient which is registered to a reference surface for verification (namely the body structure from the CT or created directly in the OST system during treatment set-up). The OST system’s calculation of the isocenter shift uses this registration result to predict the impact on the live surface position by using a volumetric deformable model. The calculated position inaccuracies are displayed live in 6D, including translational (vertical, longitudinal and lateral) and rotational shifts (yaw, pitch and roll).The OST system was tested on 7 Caucasian volunteers fixed with the 3 points open face hybrid mask and T-shaped vacuum head support (Orfit industries) to determine the accuracy of a non-coplanar treatment (using couch angles 0 o , 45 o , 90 o , 315 o and 270 o ) monitored by OST in a realistic clinical setting. Facial hair can lead to decrease in light reflection (see picture of volunteer in fig.1.). Each volunteer was monitored 3 times as in 3 consecutive treatment fractions for a real patient. In addition, these results were correlated to the couch

Conclusion This work shows that it is feasible to monitor patient positioning for a non-coplanar single isocenter SRS treatment using a commercial OST with submillimeter and subdegree accuracy without compromising the desired 1mm GTV-PTV treatment margin. Further, a regular WL test should be part of the SRS-specific QA program of a linac. PO-1600 Evaluation of a High Dynamic Multileaf Collimator for Real-Time tumor tracking C. Murillo 1 , S. Seeber 1 , P. Haering 2 , C. Lang 2 , M. Splinter 2 1 DKFZ - Deutsches Krebsforschungszentrum, Electronic & Embedded Systems Development Lab, Heidelberg, Germany ; 2 DKFZ - Deutsches Krebsforschungszentrum, Research Group e040, Heidelberg, Germany Purpose or Objective Image guided radiotherapy (IGRT) and adaptive RT are used to minimize dose deposition in normal tissue due to inter-fractional motion. Besides this, tumor tracking provides an efficient way to deposit dose to a moving target. Therefore, a Multileaf Collimator (MLC) needs to be accurate, precise and fast. Known experiments, achieving tracking with 3cm/s (isocentric) MLCs, show low dosimetric accuracy for faster organ/tumor motion. A new in-house developed High Dynamic MLC (HDMLC) has therefore been evaluated. This system uses a real-time control unit, fulfills industrial automation standards and achieves leaf speeds up to 20cm/s. Quantification of the dosimetric accuracy and dose conformity are the main goals of this study. Material and Methods The HDMLC evaluation was done using a Siemens Artiste (X-ray 6 MV) and 1D motion phantom together with EBT films (Gafchromic) and a PTW 729 detector. Experiments were made with an X1 side only HDMLC (39 x 3mm leaves) while X2 was linac formed. A magnetic encoder was placed on the phantom and acquired by the MLC controller. By placement of the phantom 45° relative to the leaf motion, X and Y movements were achieved (Fig 1). Circle and square shapes were used for the experiment. The MLC shape X movement followed the encoder signal