Purpose: To explore the feasibility of using stereotactic body radiotherapy (SBRT) to irradiate the antra of the four pulmonary veins while protecting nearby critical organs, such as the esophagus.
Materials and methods: Twenty patients who underwent radiofrequency catheter ablation for atrial fibrillation were selected. For each patient, the antra of the four pulmonary veins were identified as the target volumes on a pre-catheterization contrast or non-contrast CT scan. On each CT scan, the esophagus, trachea, heart, and total lung were delineated and the esophagus was identified as the critical organ. For each patient, three treatment plans were designed with 0, 2, and 5 mm planning margins around the targets while avoiding overlap with a planning organ at risk volume (PRV) generated by a 2 mm expansion of the esophagus. Using three non-coplanar volumetric modulated arcs (VMAT), 60 plans were created to deliver a prescription dose of 50 Gy in five fractions, following the SBRT dose regimen for central lung tumors. With greater than 97% of the planning target volumes (PTV) receiving the prescription doses, we examined dosimetry to 0.03 cc and 5 cc of the esophagus PRV volume as well as other contoured structures.
Results: The average PTV-0 mm, PTV-2 mm, and PTV-5 mm volumes were 3.05 ± 1.90 cc, 14.70 ± 5.00 cc, and 40.85 ± 10.20 cc, respectively. With three non-coplanar VMAT arcs, the average conformality indices (ratio of prescription isodose volume to the PTV volume) for the PTV-0 mm, PTV-2 mm and PTV-5 mm were 4.81 ± 2.0, 1.71 ± 0.19, and 1.23 ± 0.08, respectively. Assuming patients were treated under breath-hold with 2 mm planning margins to account for cardiac motion, all plans met esophageal PRV maximum dose limits < 50 Gy to 0.03 cc and 16 plans (80%) met < 27.5 Gy to 5 cc of the esophageal PRVs. For PTV-5 mm plans, 18 plans met the maximum dose limit < 50 Gy to 0.03 cc and only two plans met the maximum dose limit < 27.5 Gy to 5 cc of the esophageal PRV.
Conclusions: The anatomical relationship between the antra of the four pulmonary veins and the esophagus varies from patient to patient. Adding 2 mm planning margins and a 2 mm PRV to the esophagus can meet the dose constraints developed for SBRT central lung tumors. Future studies are needed to validate the safety and efficacy of the planning dose, tolerance dose to normal cardiac tissue, and adequate planning margins.
Keywords: atrial fibrillation; hypofractionation; radiotherapy; stereotactic body radiotherapy; treatment planning.