Purpose: Cardiac radioablation is used to treat ventricular tachycardia (VT) after pharmacologic and/or surgical treatment failure. Photons are primarily used since the advantages of protons are not well studied. Thus, a comparative planning study was done to assess the viability of Pencil Beam Scanning (PBS) as a treatment modality for VT.
Methods: PBS proton plans were created for 10 patients with VT previously treated with photon stereotactic body radiotherapy (SBRT). Patients were planned to a dose of 2500cGy to the planning target volume (PTV) in a single fraction. Four proton plans with three fields were created for each patient to evaluate the effectiveness of multifield (MFO) versus single field optimization (SFO) and robust versus non-robust optimization. All four plans were compared to the clinically delivered SBRT plan to gauge each method’s effectiveness in meeting the target coverage and organ at risk (OAR) objectives. Robust optimization uncertainty parameters for isocenter shift and calibration curve error were ±0.5cm and ±3.5% respectively. Robust optimization was evaluated by comparing the 2nd worst [PTV2500cGy D95%>=95] perturbation for each proton plan.
Results: Comparing average target coverage for the PTV, robustly optimized proton plans had a difference of +3.66±0.87% to 95% of the PTV, and non-robustly optimized plans had difference of -1.56±1.23%. Robust MFO plan target coverage was +3.81±0.92% and robust SFO target coverage was +3.50±0.87% compared to photon plans. PBS plans reduced OAR dose in most cases, but greater vessel and stomach max doses were increased on average by 141.2±104.2cGy and 151.0±196.4cGy, respectively with proton plans.
Conclusion: Robustly optimized PBS proton plans provide comparable target coverage to photon SBRT plans and can significantly decrease OAR dose in the mediastinum. With limited cardiac motion management techniques for PBS proton therapy, evenly weighted, robustly optimized SFO plans present a viable alternative to photon SBRT for treatment of VT.