ELECTRONIC PORTAL IMAGING WITH ONLINE CORRECTION OF SETUP ERROR IN THORACIC IRRADIATION - CLINICAL-EVALUATION

Purpose: To analyze setup errors and the feasibility of their on-line correction using electronic portal imaging in the irradiation of lung tumors. Methods and Materials: Sixteen patients with lung cancer were irradiated through opposed anteroposterior fields. Localization images of anteroposterior fields were recorded with an electronic portal ima ging device (EPID). Using an in-house developed algorithm for on-line comparison of portal images setup errors were measured and a correctio n of table position was performed with a remote couch control prior to treatment. In addition, residual errors were measured on the EPID ver ification image. Global and individual mean and standard deviation of setup errors were calculated and compared. The feasibility of the proc edure was assessed measuring intra-and interobserver variability, infl uence of organ movement, reproducibility of error measurement, the ext ra time fraction needed for measuring and adjusting and the fraction o f dose needed for imaging.Results: In two setups the procedure could n ot be finished normally due to problems inherent to the procedure. The reproducibility, intraobserver variability, and influence of organ mo vements were each described by a distribution with a mean value less t han or equal to 1 mm and a standard deviation (SD) of less than 1.5 mm . The interobserver variability showed to be a little bit larger (mean : 0.3 mm, SD: 1.7 mm). The mean time to perform the irradiation of the anteroposterior field was 4 +/- 1 min. The mean time for the measurem ent and correction procedure approximated 2.5 min. The mean extra time fraction was 65 +/- 24% (1 SD) with more than half of this coming fro m the error measurement. The dose needed for generation of EPID images was 5.9 +/- 1.4% of total treatment dose. The mean and SD of setup er rors were, respectively, 0.1 and 4.5 mm for longitudinal and -2.0 and 5.7 mm for transversal errors. Of 196 measured translational errors 12 0 (61%) exceeded the adjustment criteria. For individual patients syst ematic and random setup errors can be as high as, respectively, 15.8 a nd 7.5 mm. Mean residual error and SD were for longitudinal direction 0.08 and 1.2 mm and for transversal direction -0.9 and 1.0 mm (pooled data). For individuals, the mean residual errors were smaller than 1 m m, with a typical SD per patient of less than 2 mm. Conclusion: Setup errors in thoracic radiation therapy are clinically important. On-line correction can be performed accurately with an objective measurement tool, although this prolongs the irradiation procedure for one field w ith 65%. (C) 1998 Elsevier Science Inc.