Room: Exhibit Hall
Purpose: To provide qualitative analysis of the delivery accuracy and clinical impact of three planning methods used in lung SBRT in the presence of significant respiratory target motion.
Methods: An Eclipse treatment planning system (version 13.7, Varian Medical Systems, Palo Alto, CA) was used to create 3D conformal arc, 3D conformal arc with â€˜arc in arcâ€™, and VMAT SBRT plans on a QUASAR (Modus Medical Deivces, Ontario, CA) motion/heterogeneity phantom insert for the Arc Check device (Sun Nuclear Corporation, Melbourne, FL). The phantom was set to provide clinically relevant amplitude sinusoidal motion (1.4 and 3.0 cm) of a 3 cm spherical target embedded in a wood substrate. A 4DCT was acquired and contouring was performed on a MIP reconstruction scan to create an ITV and a 5 mm symmetric margin was used to create a PTV. Treatment plans were delivered on a Varian TrueBeam with CBCT guidance. Dosimetric analysis was performed with radiochromic film, ion chamber, and the diode array of surrounding the motion insert.
Results: In all three types of plans reviewed it was determined that the targeted region (GTV plus 5 mm margin) received the full prescription. In the direction of motion, dose in the peripheral areas of PTV decreased, with falloff region becoming larger with increased motion amplitude. Enhanced motion correlation artifacts in the dose distribution were not seen when comparing 3D conformal to VMAT.
Conclusion: All three planning and delivery methods were found to deliver the prescription dose to the target region despite significant respiratory motion. For larger motion, integral dosing of surrounding structures becomes more evident, making breath holds or gating more dosimetrically advantageous options.