Dosimetric Impact of VMAT Delivery and Target Motion: MIP-Based ITV Vs. ITV From Contoured Phases of 4DCT
M Chan*, J Li, C Burman, K Schupak, P Parhar, Memorial Sloan-Kettering Cancer Center, Basking Ridge, NJSU-C-108-2 Sunday 1:00PM - 1:55PM Room: 108
Purpose: It has been a concern that the interplay effect in dynamic radiation treatments of moving targets may compromise dose coverage. In this work we study the interplay-induced dosimetric impact to moving targets in VMAT delivery.
Methods: A new patient-specific QA system with ArcCHECK/3DVH v3 (Sun Nuclear Corp) was utilized to estimate "true" dose to a moving target during VMAT delivery. A set of 4DCT scans was acquired for an SBRT lung patient and a VMAT plan was generated using an in-house TPS with ITV (internal-target-volume) generated from the MIPs (maximum-intensity-projections) of 4DCT. The prescribed dose to the PTV (ITV+5mm) was 45Gy in 5 fractions. The motion trajectory was obtained based on the GTVs (gross-tumor-volume) from the 10 phases of 4DCT. The QA system was first used to measure the dose distribution delivered according to the plan. Then, the motion trajectory was incorporated into the QA algorithm to derive the "true" dose coverage, with the virtual motion simulation.
Results: The MIP-based ITV had measurable difference from the ITV generated with contoured phases of 4DCT. The dose differences for targets and critical structures between measurements and TPS were: -1.9%, PTV(D95); -3.5%, ITV(D95); 3.0%, max cord; -1%, mean heart; -1.2%, mean esophagus; 1.6%, mean right lung; -0.9%, mean left lung. After accounting for the statistical mean of motion-simulated dose for 5 fractions, the dose differences for the ITV(D95) became -4.1%. However, the dose coverage of the moving GTV itself is nearly unchanged from the plan PTV coverage (<0.2%), indicating that the 5mm margin was adequate to compensate for dose perturbation due to motion.
Conclusion: The interplay between organ motion and dynamic radiation delivery leads to dose coverage differences between treatment delivery and the static ITV based treatment plan. The new patient-specific QA device may be a useful tool to estimate the differences.