Variation in Lung Tumour Breathing Motion between Planning Four-dimensional Computed Tomography and Stereotactic Ablative Radiotherapy Delivery and its Dosimetric Implications: Any Role for Four-dimensional Set-up Verification?

TBM variation between 4DCT and treatment and between SABR fractions was modest for most patients. However, 19% of patients experienced significant TBM variation that could be clinically relevant for those most severely affected. It seems prudent to carry out on-couch assessment of TBM at each SABR fraction to identify such patients who might benefit from respiratory gating or adaptive radiotherapy to maintain tumour motion within the planned limits.