Towards MRI guided radiotherapy for lung cancer

Abstract

Magnetic resonance imaging (MRI) is increasingly being incorporated in radiotherapy (RT) planning due to superior soft tissue contrast compared to CT, however its use for lung cancer RT has been limited. This thesis addresses the feasibility and potential benefit of MRI for lung cancer RT. A literature review on the feasibility of MRI in lung RT identified that a combination of T2 and T1 weighted images can demonstrate tumour infiltration and mediastinal nodal involvement. Using this information, a free-breathing protocol was developed to aid target volume delineation and overcome challenges with breath-hold non-compliance. An assessment of a radiologist-led workshop on inter-observer variability in volume delineation showed no significant improvement. To assess the potential benefit of MRI for lung RT, a prospective clinical trial was conducted. Inter-observer variability on target volume delineation for T2 MRI and CT was assessed. Volumes delineated on CT and T2 datasets showed significant differences in overlap measures (p<0.01). However inter-observer variability within each imaging modality demonstrated good agreement between observers (dice similarity coefficient ≥0.7), indicating that replacement of CT with T2 MRI did not have a negative impact on interobserver variability. Assessment of motion between 4DCT and cine MRI was investigated. Tumour motion between 4DCT and 2D cine MRI was shown to be similar, however cine MRI showed larger variation in superior inferior motion for individual patients. Cine MRI can potentially be used as a complementary imaging tool to 4DCT for motion definition. Potential application of MRI for adaptive RT (ART) was investigated by evaluating tumour changes on sequential imaging. Volumes delineated on cone beam CT and MRI were significantly different. Rate of primary tumour regression was shown to be similar, but nodal regression was greater on MRI. While further work is necessary to demonstrate the benefit of MRI to CBCT and CT, the results of this study provide initial data for incorporation of MRI for ART. This thesis suggests that MRI has the potential to enhance lung cancer RT. However the differences demonstrated between MRI and current imaging modalities needs further investigation prior to any clinical implementation.