Evaluation of tumour motion and internal/external correlation in lung SABR

Objective: This study aims to analyse lung tumour motion and to investigate the correlation between the internal tumour motion acquired from four- dimensional computed tomography (4DCT) and the motion of an external surrogate. Methods: A data set of 363 4DCT images was analysed. Tumours were classified based on their anatomical lobes. The recorded gross tumour volume (GTV) information included the centroid GTV motion in the superior-inferior, anteroposterior and left- right directions, and in three- dimensions (3D). For the internal/external correlation, the RPM surrogate breathing signals of 260 patients were analysed via an in- house script. The external motion was correlated with the 3D centroid motion, and the maximum tumour motion via Spearman's correlation. The effect of tumour volume on the amount of motion was evaluated. Results: The greatest 3D tumour amplitude was found for tumours located in the lower part of the lung, with a maximum of 26.7 mm. The Spearman's correlation of the internal 3D motion was weak in the upper (r = 0.21) , moderate in the middle (r = 0.51) and the lower (r = 0.52) lobes. There was no obvious difference in the correlation coefficients between the maximum tumour displacement and the centroid motion. No correlation was found between the tumour volume and the magnitude of motion. Conclusion: Our results suggest that tumour location can be a good predictor of its motion. However, tumour size is a poor predictor of the motion. Advances in knowledge: This knowledge of the distribution of tumour motion throughout the thoracic regions will be valuable to research groups investigating the refinement of motion management strategies.