Physics and Astronomy Colloquium Tuesday, April 16, 2002 4:10 p.m., Room 118 Physics-Astronomy Building Refreshments served outside Room 224 Physics-Astronomy Building (Student Informal Meeting will be held from 3:15-3:45 in Room 224 PA) "Image Guided Radiation Therapy" Jon J. Kruse Mayo Clinic Most courses of external beam radiation therapy (EBRT) are administered through 20-40 daily treatment fractions. Each day the patients are aligned to the treatment machine via external marks on their skin, under assumption that the daily positions of cancerous lesions and normal tissues correlate with the external marks. The traditional method for verifying geometric accuracy of EBRT involves placing a film cassette behind the patient and using radiation from the treatment accelerator to generate an image of the field portal and the patient's skeletal anatomy. An alternative to portal filming is the use of an electronic portal imaging device (EPID), which is an array of radiation detectors mounted behind the patient in place of the film cassette. EPIDs offer a number of advantages over film, including computer-controlled acquisition, extended dynamic range, real-time image display, and software-assisted analysis. This presentation will describe various EPID detector types and give examples of clinical applications. Frequent imaging with an EPID allows users to monitor patient setup variation or organ motion within the patient on a daily basis. Movie-loop acquisition modes can monitor dynamic treatments or measure target motion during treatment. Real-time feedback allows for on-line correction of target alignment, based on bony anatomy or visualization of target tissue. Finally, the quantitative nature of an EPID image can be exploited to perform ex-vivo dosimetry or the measurement of dose exiting the patient and comparison to planned distributions.