AbstractID: 7859 Title: A program to automatically determine motion during radiotherapy Organ motion during the delivery of radiotherapy can often be visualized directly when using high-resolution, fast-readout EPIDs. We developed a computer program incorporating a Sobel edge operator to automatically process the images and to display the region where movement was detected. A silicon amorphous detector was used to acquire portal images at a rate of up to 210 frames/minute during treatment. The images were converted into DICOM and processed using routines available in the Interactive-Data-Language (IDL) software. The contrast within the portal images was enhanced using histogram equalization and edge enhancement was performed using a Sobel operator. The individual image frames were added together to obtain a summed image. Pixels with intensifies within any frame that differed by more than ±11% from the mean (corresponding to 1.5× ZHUH KLJKOLJKWHG RQ WKH GLVSOD\HG VXP RI DOO IUDPHV RI WKH SRUWDO LPDJH 7KH VRIWZDUH LV YHU\ VXFFHVVIXO DW LGHQWLI\LQJ motion along a boundary between regions with substantial differences in pixel values but lowering the difference threshold too much makes the algorithm find too many regions of apparent motion that could be associated with Poisson noise in the image. A human observer is better than the presented threshold algorithm at detecting anatomical motion in regions of the image with slight differences in pixel intensities but an automatic program is useful to analyze the large number of images that are collected during a course of treatment.