Magnetic resonance imaging thoracic organ-at-risk atlas for radiation oncology

Background and purpose: The use of Magnetic Resonance imaging (MRI) for radiotherapy (RT) planning for locally advanced non-small cell lung cancer (LA NSCLC) could improve RT precision due to its superior soft tissue definition compared to computed tomography (CT). However, thoracic oncologists have limited experience of identifying thoracic structures on MRI. The aim of this study was to provide recommendations for MR sequences for thoracic organ at risk (OAR) contouring and present an atlas and descriptive instructions for delineation of thoracic OARs in the setting of MRI-guided radiation treatment planning and guidance. Materials and methods: MRI scans were acquired in nine patients with early-stage lung cancer on a diagnostic 1.5 Tesla system. MRI sequences included T1-weighted and T2-weighted imaging techniques, each optimised for visualisation of particular OARs. OAR delineations were carried out and reviewed by an international panel of thoracic oncologists and MR radiologists. Results: Thoracic MRI OAR contouring recommendations and atlas were developed by multi-institutional collaboration of six radiation oncologists and two MR radiologists. The atlas and contouring recommendations are described alongside high-resolution contoured MR images. Conclusions: This consensus MRI contouring atlas has a variety of potential applications, from integration of MRI within the standard CT-based workflow in order to improve the contouring accuracy of challenging structures such as the brachial plexus, to forming the foundation of an MRI-only workflow for use in MRI-guided treatment machines. This guideline should provide a useful reference for education and will facilitate uniformity in MRI-based contouring of OARs.