Dosimetric consequences of inter-fraction breathing-pattern variation on radiotherapy with personalized motion-assessed margins

The data from eight patients who had undergone stereotactic body radiotherapy were selected due to their 4D-CT planning scans showing that their tumours had respiratory induced motion trajectories of large amplitude (greater than 9 mm in cranio-caudal direction). Radiotherapy plans with personalized motion-assessed margins were generated for these eight patients. The margins were generated by inverse 4D planning on an eight-bin phase-sorted 4D-CT scan. The planning was done on an in-house software system with a non-rigid registration stage being completed using freely available software. The resultant plans were then recalculated on a 4D-CT scan taken later during the course of treatment. Simulated image-guided patient set-up was used to align the geometric centres of the tumour region and minimize any misalignment between the two reconstructions. In general, the variation in the patient breathing patterns was found to be very small. Consequently, the degradation of the mean dose to the tumour region was found to be around a few percent (<3%) and hence was not a large effect.