The purpose of this study was to develop electronics portal imaging device (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 in water at depth of 10 cm with our algorithm included three parameters (linearity of dose response with MU, beam profile correction, and scatter kernel). Image pixel values 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. To convert scatter dose distribution from EPID plane to water plane at depth of 10 cm, scatter kernel was applied to
EPID images. The algorithm accuracy was validated with 36 fields of 10 VMAT plans by using gamma analysis comparing between our EPID-based dosimetry and plane dose distribution of treatment planning systems (TPS). Gamma passing rates (GPR) were determined with criteria of 2%, 2 mm. Mean of GPR was 97.74%, lowest was 93.60%, and the highest was 99.98 %. Our EPID based dosimetry showed good agreements with plane dose distribution in water. These results indicated that our EPID-based dosimetry can perform FFF-beam verification.