Room: ePoster Forums
Purpose: Evaluate the performance of a gantry-resolved EPID-based QA method for patient-specific QA of large VMAT fields using process-based limits per TG-218.
Methods: A gantry-resolved EPID-based QA method was used for patient-specific QA of 35 large VMAT fields (field size range from 17x30cm2 to 30x40cm2) used for GYN cancer treatments. All fields were measured with two EPID models and two photon energies, accounting for a total of 140 measurements divided into 4 different groups (processes). The method created pseudo-3D dose distributions from stacked portal images acquired on TrueBeam Linac using dosimetry (integrated) acquisition. Predicted portal dose distributions were calculated based on MU information contained in the image headers. Gamma pass rates (GPRs) for pseudo-3D and composite 2D dose distributions were used to calculate process-based tolerance and action limits following the TG-218 methodology (3%/2mm/10%threshold criteria). For the pseudo-3D distribution, the gantry angle accuracy requirement is given by an angle-to-agreement (ATA) criteria of 3â?°.
Results: All gamma pass rates for gantry-resolved pseudo-3D distributions were within the recommended universal action limit of 90%, and average pass rates were higher than 95% for all processes. Gamma analysis of 2D distributions produced average gamma pass rates close to 99% and an overall performance superior to Portal Dosimetry. All calculated tolerance limits were stricter than the action limits and no processes displayed an out-of-control behavior. Different variations were observed among the processes, indicating that further improvements in the processes are possible. The calculated process-based tolerance and action limits complied with TG-218 recommendations.
Conclusion: The method has shown its suitability for use in patient-specific QA of large VMAT fields. The results meet the universal action limits recommendation of TG-218. The variability among different processes indicates that improvements are possible to obtain more suitable process-specific tolerance and action limits to evaluate our results.