A low-cost, flexible, and accurate technique to study new immobilisation systems for radiotherapy and their 2D in-plane positional reproducibility and stability

Recently, there has been a resurgence of interest in upright, gantry-less radiotherapy due to its potential to reduce the costs associated with new treatment rooms and improve comfort/clinical outcomes for specific patient groups. Consequently, new patient positioning and immobilisation devices are being developed for upright and conventional radiotherapy. Before any new RT immobilisation system is implemented clinically, pre-clinical tests should be performed to verify its reproducibility and stability. Surface-guided radiotherapy (SGRT) can be used; however, commercial SGRT systems are expensive and can require complex installation and calibration procedures. Therefore, we developed a low-cost, flexible 2D in-plane displacement measurement system to support the testing and development of new immobilisation devices for RT. The method developed in this work couples ArUco markers with regular cameras positioned orthogonally on tripods to capture images in two planes. Markers fixed onto human subjects can then be used to assess the inter- and intra-fractional repeatability and stability associated with new immobilisation systems. A Python algorithm was developed to automatically detect ArUCo markers in each camera image and infer the distance between markers across multiple camera images. Contrary to most measurement systems, the proposed method does not require camera calibration, and any camera with sufficiently high image resolution and relatively low lens distortion can be used. Rigorous validation experiments, using simulated data, were performed to test the algorithm's measurement accuracy for different marker separations and camera-to-marker distances. Accuracy validation test results show that the proposed system has a mean measurement error below 1mm. These errors are at the same order of magnitude as the commercial SGRT systems. The proposed system provides a viable low-cost solution for assessing positional repeatability and stability of new immobilisation devices during pre-clinical development. This is particularly relevant as new positioning systems are being developed for both upright and conventional supine RT treatments.