Evaluation of the feasibility of cardiac gating for SBRT of ventricular tachycardia based on real-time ECG signal acquisition

Purpose To investigate the feasibility of cardiac synchronized gating in stereotactic body radiation therapy (SBRT) of ventricular tachycardia (VT) using a real-time electrocardiogram (ECG) signal acquisition. Methods and materials Stability of beam characteristics during simulated ECG gating was examined by developing a microcontroller interface to a Varian Clinac iX linear accelerator allowing gating at frequencies and duty cycles relevant to cardiac rhythm. Delivery accuracy was evaluated by measuring dose linearity with an ionization chamber, and flatness and symmetry with a two-dimensional detector array, for different gating windows within typical human cardiac cycle periods. To establish a practical method of gating based on actual ECG signals, an AD8232 Heart Monitor board was used to acquire the ECG signal and synchronize the beam delivery. Real-time cardiac gated delivery measurements were performed for a single 10 x 10 cm(2) field and for a VT-SBRT plan using intensity-modulated radiation therapy (IMRT). Results and discussion Dose per monitor unit (MU) values were found to be linear within most gating windows investigated with maximum differences relative to non-gated delivery of 10 MUs. Beam profiles for both gated and non-gated modes were also found to agree with maximum differences of 0.5% relative to central axis dose for all sets of beam-on/beam-off combinations. Comparison of dose distributions for intensity-modulated SBRT plans between non-gating and cardiac gating modes provided a gamma passing rate of 97.2% for a 2% 2 mm tolerance. Conclusions Beam output is stable with respect to linearity, flatness, and symmetry for gating windows within cardiac cycle periods. Agreement between dose distributions for VT-SBRT using IMRT in non-gated and cardiac cycle gated delivery modes shows that the proposed methodology is feasible. Technically, gating for delivery of SBRT for VT is possible with regard to beam stability.