ESTRO 2020 Abstract book

S920 ESTRO 2020

PO-1598 Investigating the use of RayPilot for motion management during prostate SBRT: initial experience M. Trainer 1 , B. Nailon 2 , L. Carruthers 1 , M. Kirby 3 1 Edinburgh Cancer Centre, Oncology Physics, Edinburgh, United Kingdom ; 2 University of Edinburgh, School of Engineering, Edinburgh, United Kingdom ; 3 Liverpool University, Department of Radiotherapy, Liverpool, United Kingdom Purpose or Objective Discrepancies between the position of a patient on a planning CT and their treatment position can impact the dosimetry of their radiotherapy treatment, especially for prostate SBRT 1 . Real-time positional verification is therefore an important consideration in the safe delivery of SBRT. Here we perform an initial assessment of the viability of the RayPilot system (Micropos Medical, 2019) for intra-fractional motion tracking for our SBRT technique and benchmark it against local imaging protocol using kV planar imaging and CBCT, focusing on the first three The RayPilot real-time tracking system consists of a table- top array containing an antennae, and a small transmitter inserted trans-perineally into the prostate. Here it was configured on a Varian Truebeam linear accelerator (Varian Medical Systems, Palo Alto, CA, USA). For our first 3 prostate cancer patients, SBRT treatment plans were prepared on the Eclipse Treatment Planning System (v.13.6) following the PACE trial protocol (36·25 Gy in 5 fractions (fx) over 1–2 weeks). The main imaging method used for positional verification was kV orthogonal pairs matched to fiducial markers with additional pre & post treatment CBCTs. In parallel, the RayPilot system was used to monitor changes in the transmitter position during treatment with the beam halted if the positional discrepancy was more than 0.2 cm. The 3-D difference in the position of defined measurement points following image registration in Eclipse was recorded and analysed. Results The mean displacement of the transmitter was below the local imaging threshold tolerance (0.2cm) for the first 2 patients (Table 1). The mean CT (1.2cm) & CBCT (-0.24cm) displacements for Patient 3 were outside the imaging tolerance in the z-direction. In Table 2 the CT Vs CBCT displacement was found to be more than 0.2 cm in x,y & z: 0%, 6.3%, 56.3% based on RayPilot; and 14.6%, 11.5%, 25% based on fiducials. The pre vs post CBCT displacements were 5.6%, 11.1%, 27.8% based on RayPilot and 1.9%, 7.4%, 11.1% based on seeds. patients in the study. Material and Methods

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