In Step 2 (Figure 1, Right panel), an electrophysiologist and radiation oncologist collaborated to select target areas based on the combined 17-segment model that was created in Step 1. Regions suspected to be the substrates or foci were designated as ON (red), unselected regions were designated as OFF (green), and organs at risk of failure were designated as OAR (yellow). OARs must be considered, including cardiac substructures (e.g., coronary arteries, valves, papillary muscles, and stimulatory conduction systems) and surrounding organs (e.g., the stomach wall, ribs, esophagus, trachea, and spinal cord). For the tentative target, the medical physicists used radiotherapy planning software (Eclipse ver. 13.7; Varian Medical Systems Inc., Palo Alto, CA, USA) to contour the cardiac substructures and target volume.16 (link)
In step 3, the irradiation volume and the dose distribution was estimated using an algorithm of Acuros XB (Varian Medical Systems Inc.). Adjustments were required if the dose calculation for critical organs exceeded the constraint. Targeting and contouring are mutually repeated many times to improve the accuracy of the treatment plan and to determine the final irradiated area. Considering heart rate and respiratory variability, as well as rotational errors,17 (link) a margin of 2–5 mm was expanded to the clinical target volume (CTV), defined as the internal target volume and planning target volume (PTV). The amount for the PTV should be <100 mL, based on a previous report (mean 98.9 mL, range 60.9–298.8 mL).3 (link)