SU‐E‐T‐109: Evaluation of the Stability of the Central Axis Response of An Amorphous‐Silicon EPID Used in the Continuous Mode of Image Acquisition

Purpose: To investigate the stability of the central axis (CAX) response of an amorphous‐silicon (a‐Si) EPID in continuous mode as function of the number of frames per image and the dose rate of dose delivery. Methods: The Varian aS1000 amorphous‐silicon EPID (PortalVision, Varian Medical System, Palo Alto, CA) fixed to a Novalis TXTM (Varian Medical System, Palo Alto, CA) is used in this study to investigate the CAX of the EPID with time. The continuous mode of acquisition is capable of acquire a sequence of multiple images in real time during radiation is being delivered. A 10×10 cm2 field is imaged by an onboard EPID imaging system for 100 MU using the continuous mode acquisition varying the number of frames (frame rate) contributing each image. The source to detector distance (SDD) in fixed at 110 cm for all measurements. Accelerator output is imaged for dose rates of 600 and 300 MU/min. The frame rate is varied from 1 to 10 frames contributing to each image. The rate is approximately 7.5 frames per second. The EPID images were analyzed using an in‐house routine developed in MATLAB V7.8.0347 (The Mathworks. Inc), this consists in evaluates the response of a central region of the images which consist of 50×50 pixels in the central axis region. Results: After the analysis of the images the results show a period of over‐response followed by a period of under‐response during the first 2 seconds of the irradiation. After 3 seconds EPID response settles to nearly constant value. The phenomenon is nominal behavior for a linear accelerator. Conclusions: This works shows that the central axis response with time for an a‐Si EPID using the continuous mode of acquisition is accurate to ∼1/7 of a second.