SU‐E‐T‐456: Development of An EPID‐Based Output Measurement and Dosimetric Verification Tool for Electron Beam Therapy

Abstract

Purpose:To develop an efficient and robust method for output and absolute dosimetric measurements of electron beam therapy by using a high spatialresolution and high frame‐rate amorphous silicon flat‐panel electronic portal imaging device (EPID).Methods:A previously‐established EPID dosimetry system was extended to measure the output factors of electron beams of various sizes and their planar dose distributions at reference depths. Specific EPID responses to different electron energies were derived from Monte Carlo (MC) simulations to de‐convolve the EPID raw images to the incident electron fluence map. To reconstruct the 2D water‐based dose distribution map at reference depths for different energies, the fluence map was further convolved with a MC simulated pencil beam kernel. Different energies of 6, 9, 12, 16, and 20 MeV from a Varian C‐series linac were tested. Standard square fields ranging from 2×2 to 15×15 cm2 were measured and validated against film/ion chamber measurements. Small fields of different patient cases with irregular cutout shapes were also tested.Results:The EPIDmeasured output factors for 2×2, 3×3, 6×6, and 10×10 cm2 fields with all electron energies agree with the film/ion chamber data within 2.7%. The average discrepancy between EPID and film/ion chamber measurements was 1.4%. The differences for the three patient cases were 0.1%, 2.1%, and 1.6%. Deliveries with different monitor units were tested and the results exhibited good linearity. Measurements with different dose rate showed that the dose rate dependence was less than 1%. Using the proposed method, 2D absolute dose maps were created from the EPID raw images and the results were consistent with film measurements.Conclusion:The efficient data readout and portability of the newly developed EPID system provides an efficient and accurate solution for electron beam therapy. It addresses an important unmet clinical need for a fast and reliable small field output factor measurement.