Gated Therapy Dosimetric Quality Assurance with the Radiological Physics Center (RPC) Lung Phantom Reciprocating Platform Assembly
K Li1, 2, J Newton1, 2*, (1) John R Marsh Cancer Center, (2) Associates in Medical Physics, Hagerstown, MarylandSU-E-P-13 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To investigate the effect of motion in gated therapy for different treatment plans with the Radiological Physics Center (RPC) Four Dimension (4D) Quality Assurance (QA) Lung Phantom Assembly System.
Methods:A RPC lung phantom went through a 4D scan with GE Simulation CT. The Phase signal was recorded by converting the longitudinal signal into vertical motion through a pulley structure. The images were then exported to the Eclipse treatment planning system. With the reciprocating platform, a seven-field conformal plan and a seven-field IMRT plan were generated in 6MV SRS mode. All plans satisfied the target definition and dose prescription and constrains requirement by the RPC. The 30x30x12cm solid water phantom was sent through the same scanning procedure as the RPC lung phantom and used as a dosimetric check for the different plans. Plans utilizing a 5cm diameter target were used to find the optimal gated window, and then a 2cm diameter spherical target plan and the two RPC QA plans were delivered to verify the gated window effect. The similarity of plans dose and film results were evaluated by analyzing the penumbra of the profile along y directions with RIT Dosimetry System.
Results:For a single IMRT field targeting a 5cm diameter sphere, when the plan was delivered to a target at rest, and at motion with a 20%, 40%, 60%, 80% and 100% gating window, the corresponding the penumbras increased from 102%, 119%, 182%, 235% and 266%. For the 2cm spherical target conformal plan, the 3 dimensional conformal QA plan and IMRT QA plan, the penumbra ratio between the 20% gated window and static phantom were 108%, 107% and 109%.
Conclusion:The 20% gated window gives acceptable dosimetric results for planning utilizing a 5mm margin. The complexity of dosimetry due to arbitrary directional motion in a real patient requires further investigation.