Development of EPID-based dosimetry for FFF-beam verification in radiation therapy

The purpose of this study was to develop Electronic Portal Imaging Devices (EPID)based dosimetry for Flattening-Filter-Free (FFF) beam verification. All radiation measurements were performed with source to imager distances (SID) of 150 cm to reduce saturation effect. EPID images were converted to radiation absorbed dose with our algorithm including four parameters: linearity of dose response with Monitor Unit (MU), beam profile correction, collimator scatter, and scatter kernel. The Calibration Units (CU) of image were scaled to dose ( Gy) by using linearity of dose response with MU. Off-axis response differences between EPID and water were reduced with beam profile correction. Scatter kernel was applied to EPID images to reduce the residual error. The algorithm accuracy was validated with 12 arcs of Volumetric Modulated Arc Therapy (VMAT) plans by using gamma analysis comparing between EPID-based dosimetry and a plane dose distribution of Treatment Planning Systems (TPS). Gamma Passing Rates (GPR) were used to determine the dose agreements with criteria of 2%, 2 mm and 3%, 3 mm. The mean of GPR was 97.91%, and 99.62% for criteria of 2%,2 mm and, 3%, 3 mm, respectively. Our EPID-based dosimetry showed good agreement with plane dose distribution in water. These results indicated that our EPID-based dosimetry can perform FFF-beam verification.