Measurement of patient set-up variations using an electronic portal imaging system

Electronic portal imaging devices (EPID) have the potential to be used in on-line verification and correction of patient positioning during radiotherapy treatments. On-line image processing is facilitated by the speed of the image acquisition of EPIDs and the image enhancement facilities available with the accompanying software packages, The purpose of this study was to assess the reproducibility of patient positioning during radiotherapy treatments, Megavoltage portal images were obtained using the Varian PortalVision(TM) detector, a fast matrix ionization portal imaging device, attached to a Clinac 2100C linear accelerator by a retractable arm. Portal images were acquired (2-4 per treatment field) for treatment sites including head and neck. chest, abdomen and pelvis. Reference structures for specific treatment sites were established and defined on the reference images. These structures were used to perform on- and off-line assessment of patient movement during the course of treatment by matching anatomy on the reference image with the subsequent portal images of the same treatment field. Reliable millimetric measurements were obtained from the PortalVision(TM) only when the irradiated volume contained identifiable fixed bony structures. In the head and neck region small variations of patient position relative to field edge, with standard deviations of less than 3 mm, were observed. These deviations are small because of the use of fixation shells. In the pelvic and abdominal regions, larger set-up variations over 5 mm were observed. In cases where set-up deviations of greater than 6 mm occurred, off-line assessment of patient set-ups was carried out with a view to improving the position of the patient in the next treatment session. The results show the need for better patient fixation, especially for patients receiving treatment with multileaf collimator fields where field margins have been reduced.