Recommendations for contouring of gross tumour volume for locally advanced lung cancer using magnetic resonance imaging

Background and purpose: The use of Magnetic Resonance imaging (MRI) for radiotherapy planning and guidance for locally advanced non-small cell lung cancer (LA NSCLC) is novel. The superior soft tissue definition of MRI compared to CT, may facilitate more accurate gross tumour volume (GTV) definition, with the goal of improving radiotherapy precision. This work aims to develop GTV contouring recommendations for NSCLC on MRI. Materials and methods: Two international training workshops on GTV delineation for LA NSCLC were attended by thoracic radiation oncologists and MR radiologists. Thoracic radiation oncology experts contoured nine cases of LA NSCLC, firstly, on mid-position 4D-CT with PET-CT guidance, and secondly on non-contrast MRI, registered with the CT and PET-CT. Consensus contours generated on CT and MRI were discussed and finalised during two international meetings. Results: Recommendations on GTV delineation for LA NSCLC for both the primary tumour and individual lymph node stations using thoracic MRI were produced and are provided in this document. Consensus contours generated on CT and MRI for specific clinical scenarios were demonstrated and challenges addressed. Conclusions: We provide the first set of consensus recommendations on GTV contouring on MRI for LA NSCLC through an international collaborative process between international experts in thoracic radiation oncology and MR radiology. This work provides an initial step towards standardisation of lung GTV delineation on MRI, which is necessary prior to any meaningful assessment of the benefits of MRI in GTV definition compared to current practice.