SU-F-T-541: Impact of VMAT Dose Calculations with Respiratory Movements in Lung.
PURPOSE
To evaluate dose impact due to respiratory movements for volumetric modulated arc therapy (VMAT) in lung, using deformation technique.


METHODS
The dose impact due to respiratory movements was evaluated for VMAT in lung. VMAT dose distributions were calculated with 4D-CT images for a 3-D dynamic lung phantom and seven patients. First, one-arc VMAT dose distributions were calculated at the reference phase (the maximum of expiratory phase) using the Varian Eclipse (ver.10) treatment planning system. All VMAT were planned with 48Gy for 4 fractions. The VMAT dose distributions were divided into 88 control points (CPs) and were assigned to each one of ten phases divided through one respiratory cycle time. Next, the dose distributions were recalculated at each phase and were deformed at the reference phase using DIRART ver.1.0a and Velocity AI ver.3.1 software. The deformed dose distributions were summed up. Finally, VMAT dose distributions with respiratory movements were evaluated from dose volume histograms (DVHs).


RESULTS
VMAT dose distributions deformed by DIRART and the Velocity AI in the phantom study agreed within 2% with that of the reference phase for 95% of GTV (D95 ). In the patient study, deformed VMAT dose distributions decreased up to 4.15% and 3.46% for DIRART and Velocity AI, respectively. The maximum dose of GTV (Dmax) tended to be higher with increasing tumor movements. In constant, D95 for DVHs decreased with increasing tumor movements. The discrepancy in the VMAT dose distributions deformed by two software depended on specific properties of respective algorithms and parameters.


CONCLUSION
Respiratory movements impact on the dose distributions for GTV. The Dmax may relate with amplitude of tumor movements.