ESTRO 2022 - Abstract Book

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Abstract book

ESTRO 2022

Conclusion The ML approach was able to detect markers both underneath the CT scanner’s couch top or the SRS reference point. Based on these detected points, couch parameters could be predicted with high accuracy suitable as a reliable starting point for efficient SGRT-based patient positioning. Indexing all immobilization devices is key to minimize positioning variations and to maximize accuracy.

Poster Discussion: 21: Implementation of new technology & techniques

PD-0891 Validation of EPID reference pixel for MLC and jaw calibration on a high-field MR linear accelerator

H.L. Riis 1,5 , A. Fietje 2 , A.S. Bertelsen 3 , U. Bernchou 4,6 , C. Brink 1,6 , B.C. Buthler 1

1 Odense University Hospital, Department of Oncology, Odense, Denmark; 2 Elekta Instrument AB, Costumer Service, Stockholm, Sweden; 3 Odense University Hospital , Department of Oncology, Odense, Denmark; 4 Odense University Hospital, Oncology, Odense, Denmark; 5 University of Southern Denmark, Department of Clinical Research, Odense , Denmark; 6 University of Southern Denmark, Department of Clinical Research, Odense, Denmark Purpose or Objective The Elekta Unity linear accelerator (linac) is a high-field magnetic resonance (MR) linac allowing MR image-guided adaptive radiotherapy. The Unity linac has a build-in electronic portal imaging device (EPID) mainly for quality assurance (QA) and calibrations. The position of the central axis projection onto the EPID for gantry at zero degrees is essential for many QA procedures. The current study aims to create a simple phantom capable of measuring this zero position of the EPID, which can be used as an independent check of the vendor calibration. Materials and Methods Copper spheres (CSs), Æ4 mm, were pairwise placed vertically above each other. The CSs had a hole drilled centrally (Æ0.8 mm) such that they could be placed on a sewing thread, approximately 40 cm apart. The thread was passed through the hole and glued to fasten the position of the CS. Five strings, each with two CSs, were placed in a box to prevent the impact of air circulation. One line was placed close to the expected central beam, and the four others placed in the corners of a 6 × 6 cm square surrounding the centre string, Fig. 1(a). For varying gantry angles (±1º in step 0.1º), the CSs were irradiated with 20 MU while acquiring EPID images in both clockwise (CW) and counter-clockwise rotation (CCW). The projection of each pair of CS on the EPID will be on a line passing through the point of vertical beam direction, which for zero gantry position will be the zero position of the EPID. A MATLAB code was used to analyse the images for CS positions and perform cross-point calculations, Fig. 1(b).