Background: To investigate whether a standard Cone beam CT (CBCT) scan can be used to determined the intra- and inter-fractional tumour motion for lung tumours that have infiltrated the mediastinum.
Material and methods: This study includes 23 patients with non small cell lung cancer (NSCLC). The intra-fractional tumour motion was analysed for each patient on a 4D-CT scan as well as on three 4D-CBCT (fraction 3, 10 and 20). The 4D-CBCT was reconstructed from a standard 3D-CBCT using in-house developed software. The tumour (GTV) was delineated in the first phase of the 4D-CT. Registration of phase one from the 4D-CT and 4D-CBCT was used to copy the GTV to the CBCT scans. Hereafter the motion of the outlined GTV was tracked in the planning 4D-CT and the three 4D-CBCT using Pinnacle(®) version 8.1w (research version). Additionally, the inter-fractional tumour movement, relative to the bony structure, was obtained from the difference in tumour position between the 3D-CT and the standard 3D-CBCT.
Results: It is possible to track a lung tumour with mediastinal infiltration in the 4D-CBCT scan based on a standard 3D-CBCT. The respiration motion in the 4D-CBCT is not significantly different from the result found from the initial 4D-CT. Likewise, no differences in respiration motion was found between fractions 3, 10 and 20.
Conclusion: This study shows that it is possible to track tumour motion for NSCLC patients with mediastinal infiltration using a standard 3D-CBCT. No change in the intra-fractional tumour motion of clinically relevance was observed during the fractionated treatment course. The inter-fractional tumour motion found underlines the importance of using daily IGRT with online match on soft tissue in order to be able to reduce treatment margins.