SU-D-211-06: The Effect of Motion in RapidArc Lung SBRT Delivery.

PURPOSE: RapidArc is routinely used for stereotactic radiotherapy for lung cancer. While treatment dose is optimized and calculated on a static CT image, the motion of the target in conjunction with the motion of the MLC may Result in a delivered dose deviating from the planed dose. In this study, we investigate the dosimetric consequences of the inter-play effect by simulating dynamic dose delivery on a dynamic CT dataset of real patients. METHODS: The target motion in 20 patients was analyzed and 5 patients with >10 mm motion were chosen for this study. The RapidArc plan for eachpatient is optimized on a free-breathing CT using 2 arcs. Inherent in each plan is data on the associated parameters such as timestamp, MLC leave position, gantry angle and delivered beam MUs for each control point. Simulated dynamic delivery is performed by associating these parameters with each of the breathing phases of the 4D-CT. The starting breathing phase is selected randomly for each of the two arcs. Dose from the derived partial plans associated with each phase of the 4D-CT dose is recalculated in Eclipse. Accumulation of dose is performed using deformable image registration from each phase of the 4D-CT to the exhale phase of the 4D-CT. RESULTS: The coverage of the GTV and PTV shows negligible variations from the interplay effect. But the Homogeneity Index is affected by the motion. The prescription isodose volume is smaller than what was from the treatment plan dose. There were both intra- and inter-fraction effects seen inthe OARs dose in some patients. CONCLUSIONS: We investigated the motioneffect in RapidArc Lung SBRT delivery in 5 patients. Negligible variations were shown for target coverage. However the motion effects were observed in high dose distribution and volume. Some OARs dose distributions were affected by the motion.