Cardiac radiosurgery motion management – investigation of regional myocardial motion and cardiac gating

Abstract:

Ventricular tachycardia (VT) is an abnormally rapid heart rhythm that can be life-threatening, leading to severely diminished cardiac output or sudden cardiac death. VT predominantly arises from scarred myocardium, where re-entrant electrical circuits cause continuous re-excitation. Cardiac radiosurgery is a non-invasive treatment for VT, where a linear accelerator is used to irradiate the arrhythmogenic scar region. Target motion management in cardiac radiosurgery can be challenging due to the complex combination of cardiac and respiratory motion. An internal target volume (ITV) approach is most commonly used to encompass the entire motion, but inaccurate or excessive margins may lead to suboptimal outcomes or adverse effects. There is a significant need for research towards improved motion management strategies in cardiac radiosurgery. In particular, cardiac gating has not been thoroughly investigated -- treating only during minimal motion periods of the cardiac cycle would allow for smaller treatment margins and reduced dose to surrounding areas.

The first aim of this thesis was to quantify cardiac-induced motion – cardiac magnetic resonance (CMR) images were analyzed to assess regional left ventricle (LV) motion for hearts with varied cardiac function, ranging from healthy to severely reduced ejection fraction. Data is presented for epicardial and endocardial displacement in all LV segments and for each patient group, providing reference data for improved cardiac treatment margins.

For regions of the LV with significant cardiac-induced motion, cardiac gating would be beneficial. A cardiac-synchronized volumetric modulated arc therapy (CSVMAT) proof-of-principle technique was demonstrated, where a treatment plan synchronized to an electrocardiogram (ECG) signal was delivered on a Varian TrueBeam linear accelerator. The radiation beam was successfully synchronized with a variable heart rate, and film dosimetry showed excellent dose distribution agreement with the original plan.

After demonstrating the feasibility of cardiac gating on a linear accelerator, the potential benefits of a cardiac-gated radiosurgery treatment were assessed. CMR images were analyzed to compare LV segment-specific displacements and treatment areas for gated and non-gated treatments. This work demonstrated that cardiac gating has the potential to significantly reduce treatment volumes for cardiac radiosurgery, depending on patient characteristics and the region of the heart to be treated.