Characterisation of cardiac and respiratory motion in cone-beam CT images for cardiac SABR

Abstract

Purpose or Objective Cardiac SABR is a new treatment technique for ventricular tachycardia which delivers 25 Gy in a single fraction to the errant substrate in the myocardium. Planning challenges include the motion of both the diaphragm and the heart. We propose a method to track the heart and diaphragm in order to characterise motion for radiotherapy treatment planning and to quality assure patient set-up using raw CBCT projection data. Materials and Methods Planar images acquired as part of routine 4D Elekta XVI cone beam CTs included views of the heart and diaphragm over a range of 200°. The distal electrode (pacing lead tip) of implantable cardioverter devices (ICDs) were visible in the images and provided a suitable surrogate for target motion. Code written in Python 3 was used to process the images and identify the position of the pacing lead tip and level of the apex of the diaphragm (target and OAR surrogates respectively). Motion traces were reported for the lead tip in the three cardinal axes and in the sup-inf axis for the diaphragm. Fourier analysis subsequently allowed respiratory motion to be separated from cardiac motion. Validation of this process was carried out using a ball-bearing in motion on a moveable platform. Data tracking accuracy was assessed by manual identification of positions compared with automatic tracking. Data from three patients treated at our centre were analysed for cardiac and respiratory motion. Results Phantom validation measurements showed that motion was quantified to within 0.5 mm for motion amplitudes over a clinically relevant range. For the three patients included in this study, lead tip displacement was measured to be on average (± standard deviation) 9.5 ± 2.1 mm (left-right), 9.7 ± 1.8 mm (ant-post) and 12.9 ± 2.7 mm (sup-inf). Cardiac motion alone was shown to provide approximately two thirds of the total motion (see table). Diaphragm positions were between 11 and 19 mm measured as average peak-to-trough values. The accuracy of lead tip tracking was > 82% and diaphragm tracking was > 94% for each of the patients. Conclusion A novel technique for measuring motion in the hearts of patients undergoing radiotherapy has been developed and validated. A three-patient sample has reported results in-line with motion data measured by other authors using alternative measurement approaches. Characterisation of motion for radiotherapy treatment planning and quality assurance of patient set-up have been shown to be possible for cardiac SABR using routinely acquired cone-beam CT images.