PROSPECTIVE CLINICAL-EVALUATION OF AN ELECTRONIC PORTAL IMAGING DEVICE

Purpose: To determine whether the clinical implementation of an electr onic portal imaging device can improve the precision of daily external beam radiotherapy. Methods and Materials: In 1998, an electronic port al imaging device was installed on a dual energy Linear accelerator in our clinic. After training the radiotherapy technologists in the acqu isition and evaluation of portal images, we performed a randomized stu dy to determine whether online observation, interruption, and interven tion would result in more precise daily setup. The patients were rando mized to one of two groups: those whose treatments were actively monit ored by the radiotherapy technologists and those that were imaged but not monitored. The treating technologists were instructed to correct t he following treatment errors: (a) field placement error (FPE) > 1 cm; (b) incorrect block; (c) incorrect collimator setting; (d) absent cus tomized block. Time of treatment delivery was recorded by our patient tracking and billing computers and compared to a matched set of patien ts not participating in the study. After the patients radiation therap y course was completed, an offline analysis of the patient setup error was planned. Results: Thirty-two patients were treated to 34 anatomic al sites in this study. In 893 treatment sessions, 1,873 fields were t reated (1,089 fields monitored and 794 fields unmonitored). Ninety per cent of the treated fields had at least one image stored for offline a nalysis. Eighty-seven percent of these images were analyzed offline. O f the 1,011 fields imaged in the monitored arm, only 14 (1.4%) had an intervention recorded by the technologist. Despite infrequent online i ntervention, offline analysis demonstrated that the incidence of FPE > 10 mm in the monitored and ummonitored groups was 56 out of 881 (6.1% ) and 95 out of 595 (11.2%), respectively; p < 0.01. A significant red uction in the incidence of FPE > 10 mm was confined to the pelvic fiel ds. The time to treat patients in this study was 10.78 min (monitored) and 10.10 min (unmonitored). Features that were identified that preve nted the technologists from recognizing more errors online include poo r image quality inherent to the portal imaging device used in this stu dy, artifacts on the portal images related to table supports, and smal l field size lacking sufficient anatomical detail to detect FPEs. Furt hermore, tools to objectively evaluate a portal image for the presence of field placement error were lacking. These include magnification fa ctor corrections between the simulation of portal image, online measur ement tools, image enhancement tools, and image registration algorithm s. Conclusion: The use of an electronic portal imaging device in our c linic has been implemented without a significant increase in patient t reatment time. Online intervention and correction of patient positioni ng occurred rarely, despite FPEs of > 10 mm being present in more than 10% of the treated fields. A significant reduction in FPEs exceeding 10 mm was made in the group of patients receiving pelvic radiotherapy, It is likely that this improvement was made secondarily to a decrease in systematic error and not because of online interventions. More sig nificant improvements in portal image quality and the availability of online image registration tools are required before substantial improv ements can be made in patient positioning with online portal imaging.