Room: Exhibit Hall
Purpose: To study the sensitivity of an Electronic Portal Imaging Device (EPID) in detecting delivery errors for VMAT lung stereotactic body radiotherapy (SBRT) using the Collapsed Arc method.
Methods: Baseline VMAT plans and intentional error plans were generated and previously assessed for 15 lung SBRT patients. Three types of intentional errors were introduced by modifying collimator angles and multi-leaf collimator field sizes MLCFS and MLC shifts by 5, 2, and 1 degrees or millimeters. A total of 103 plans were delivered to an Elekta Synergy Linear Accelerator EPID and compared to both the original treatment planning system (TPS) Collapsed Arc dose matrix and the measured baseline EPID dose matrix. Gamma analysis of the data were performed using the OmniPro-I'mRT (IBA Dosimetry) software and gamma criteria of 1%/1 mm, 2%/1 mm, 2%/2 mm, and 3%/3.
Results: When the error-introduced EPID measured dose matrices were compared to the TPS matrices, the majority of simulated errors were detected with gamma tolerance of 2%/1 mm and 1%/1 mm. All the MLCFS and MLC shifts errors, and 5 degree collimator errors were detected when the error plans EPID measured dose matrices were compared to the baseline EPID measured dose matrices.
Conclusion: This work shows the feasibility of a simple method using the collapsed arc technique and EPID for pre-treatment verification of lung SBRT VMAT plans. The EPID was able to detect the majority of MLC and collimator errors with sensitivity to errors depending on the gamma tolerances. Future work including a gantry inclinometer during the measurement may provide a tool and feedback to detect such potential errors.