Myocardial Target Volume: Defining a Physiologically Relevant Target Structure for Stereotactic Arrhythmia Radiotherapy
Abstract
Purpose
Stereotactic arrhythmia radiotherapy (STAR) is a promising noninvasive treatment for cardiac arrhythmias. Traditional motion expansion PTVs are well-suited to oncology but little is known about their use for STAR. We propose and evaluate a novel myocardial target volume as the dosimetric endpoint of choice.
Methods
Eleven patients were treated to 2500 cGy x 1 with STAR. Scar volumes were drawn using either a dedicated electrophysiology mapping software (N=8) or on diagnostic images (N=3). Scar was mapped onto the left ventricular (LV) myocardium based on the American Heart Association (AHA) model. Both scar volumes (scar substrate and AHA segmentation) were used to quantify myocardial target volume (MTV) = scar volume / LV myocardial volume [%]. 5 mm PTV margins were applied to physician-drawn scar.
Results
Scar volume averaged 66.1±23.5cc (N=11). In N=9 patients with diagnostic imaging compatible with myocardial contours, MTV comprised 45.2±10.4% of the LV myocardial volume using physician contours and 43.8±19.2% using AHA segmentation (Pearson, R2 = 0.51). PTV volumes averaged 207.2±91.2 cc (N=11). Median stomach and esophageal Dmax (D0.03cc) were 279.0 cGy (inter-quartile range 82.0, 914.7 cGy) and 1080.7 cGy (923, 1361.7 cGy) respectively. Dose to critical digestive structures was sensitive to myocardial target volume: stomach and esophageal Dmax both increased with increasing MTV (R2=0.41 and 0.48). OAR dose necessarily compromised PTV coverage when scar extended to the LV apex, close to digestive structures: PTV V25 Gy was negatively correlated with the number of LV apical segments in which scar was observed (R2 = 0.69). MTV was significantly greater in patients where a reduced PTVeval was required for OAR sparing (Mann-Whitney, p<0.01).
Conclusion
Myocardial target volume generally agreed between segmentation methods and drove dose to critical structures, limiting achievable PTV coverage. STAR-specific target volumes and dosimetry endpoints should be investigated for treatment planning and assessment of efficacy.