SU‐E‐T‐472: Arc Therapy Dose Reconstruction Using an A‐Si EPID‐Based System

Purpose: The increasing prevalence of an amorphous‐silicon (a‐Si) electronic portal imaging device (EPID) attached to a linear accelerator has led to heightened interest in applying EPID based dosimetry to machine and patient quality assurance (QA). As such EPID dosimetry offers the potential to fill a useful role when verifying volumetric arc therapy (VMAT) treatment plans. To deliver a planned VMAT treatment, sentinel information is transferred to the linear accelerator. This information is then interpreted by the accelerator as actions to deliver a VMAT treatment. An on‐board EPID can be used to record fluence distributions during the delivery of a treatment arc. The purpose of the present study is to examine the relationship between the angular sampling and the quality of patient dose reconstruction for VMAT treatment delivery. Methods: EPID images are acquired by direct irradiation of an aS1000 EPID using a continuous image acquisition mode (6 MV beam, 3–8 frames/image, max dose rate of 600 MU/min) for a VMAT plan with variable gantry speed and dose rate. The EPID images are then exported as DICOM‐RT images. An in‐house software system is used to convert selected EPID‐based images to planar fluence for importation into a commercial treatment planning (TPS) system. This software allows one to change the sampling set of available images for import into a TPS. Thus different sampling sets can be imported into the TPS and used for 3D dose calculation.Results: EPID based IMRT dose reconstruction is able to verify the treatment plan dose distribution and capture a reasonable plan‐versus‐QA comparison with a gamma analysis (3% maximum dose, 3 mm distance to agreement). Complex H&N plans require more angular sampling than plans such as prostate. Conclusions: An EPID can be used as a tool for clinical QA tasks and the developed system may be used for VMAT. Research sponsored by Oncology Data Systems, Inc, Oklahoma City, OK.