AbstractID: 9872 Title: Patient and organ position verification using Electronic Portal Imaging Devices Patient and organ positioning have become increasingly important in modern radiation therapy. Due to the advent and popularity of IMRT treatments margins used for treatment are getting smaller and complication rates are plummeting, The use of smaller margins necessitates accurate positioning. One of the tools to reach this objective, be it to correct for placement errors or for organ motion are electronic portal imagers (EPIDs). EPIDs can be used not only to image bony anatomy, but can be used to visualize soft tissue positioning by using volume CT scans or alternatively implanted markers. The digital nature of EPIDs allows different ways, yielding momentary data. The information is available to perform on--line corrections or adaptive analysis. to process the information in snapshots, accumulated data, combined in almost real--time and can be used stored in a database for off--line Finally, goal of the EPID--image can be to visualize, bony anatomy, soft tissue or implanted fiducial markers, allowing to reduce setup errors and organ motion. Educational objectives: 1) Attain a basic understanding of adjustment and verification rationale 2) Understand how EPIDs can be used to obtain information on bony anatomy position as well as soft tissues. 3) Learn how to compare the efficacy of two different positioning modalities. Statement: The author is partly sponsored by the Uniformed Services University of the Health Sciences through Contract Number MDA905-92-C-0009 awarded to the Henry M. Jackson Foundation for the Advancement of Military Medicine as a subcontract to Varian Inc. The content of the information does not necessarily reflect the position or the policy of the Government, and no official endorsement should be inferred.