Radiotherapy beyond cancer: Motion analysis, treatment planning and real-time target localization for non-invasive cardiac radiosurgery with MRI guidance

Type of publication:  Abschlussarbeit
Type: Master's thesis
Jahr: 2015
Monat: Februar
Schule: Universität zu Lübeck
Abriss: Atrial fibrillation (AF) is the most common cardiac arrhythmia that affects millions of patients world-wide. It is usually treated with minimally invasive, time consuming catheter ablation techniques. While recently non-invasive radiosurgery to the pulmonary vein antrum in the left atrium has been proposed for AF treatment, precise target location during treatment is challenging due to complex respiratory and cardiac motion. A MRI linear accelerator (MRI-Linac) could solve the problems of motion tracking and compensation using real-time image guidance. In this study, the motion range of the target is quantified on cardiac magnetic resonance imaging (MRI), its position is determined in real-time images and the dosimetric impact of different motion compensation strategies and related margin sizes is analyzed. Five human subjects received cardiac MRI of the left atrium. A 3D scan was acquired in inhale and exhale breath-holds, respectively, to measure the left atrial respiratory shift between the extreme breathing phases. Time-resolved 4D scans acquired in exhale showing the left atrium in multiple cardiac phases were used to quantify the extent of cardiac contraction from diastole (max. volume) to systole (min. volume). Both motion components need to be compensated during treatment and were considered in the planning stage. 30 Gy single fraction treatment plans for five different motion compensation scenarios were generated on a CT of an AF patient with a centrally located esophagus undergoing catheter ablation, representing an ideal case for cardiac radiosurgery. The target definition was similar to the ablation lesions created during catheter treatment. Safety margins from 0 mm (perfect compensation) to 8 mm (uncompensated respiratory motion and contraction) were added to the target. The target localization accuracy in an MRI-Linac was retrospectively optimized on 2D real-time MRI during free breathing by employing different template matching techniques. The technique with the highest accuracy was used to determine the 3D position of the left atrium from two orthogonal interleaved real-time MRI planes. The results indicate that real-time target localization and motion compensation are mandatory for high-dose cardiac radiosurgery to treat AF. MRI-guidance on orthogonal real-time planes could successfully be implemented and, as safety margins will likely be below 5 mm, treatment could be delivered with an MRI- Linac if the measured motion can be compensated with high accuracy. The findings of this work open the possibility of treating cardiac arrhythmia patients completely non-invasively.
Autoren: Ipsen, Svenja